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Introduction to XQuery in SQL Server 2005

SQL Server 2005
 

Prasadarao K. Vithanala
Microsoft Corporation

June 2005

Summary: This white paper provides an introduction to various features of XQuery implemented in SQL Server 2005 such as the FLWOR statement, operators in XQuery, if-then-else construct, XML constructors, built-in XQuery functions, type casting operators, and examples of how to use each of these features. Non-supported features of XQuery in SQL Server 2005 and workarounds are described in this article. It also presents three scenarios where XQuery is useful. (30 printed pages)

Contents

Introduction
The XML Data Type in SQL Server 2005
Introduction to XQuery
Structure of the XQuery Expression
Operators in XQuery
The if-then-else Construct
Constructing XML Using XQuery
Built-in XQuery Functions
Type-Related Expressions
Accessing Relational Columns and Variables
Non-Supported Features and Workarounds
Best Practices and Guidelines
XML Data Modification
XQuery Usage Scenarios
Conclusion

Introduction

XML was developed for use as a document format. However, the additional features of XML, such as extensibility, support for internationalization, the ability to represent both structured and semi-structured data, and easy readability by humans and machines, have rendered XML an immensely popular platform-independent data representation format. As XML gains wide acceptance, users apply XML to solve complex business problems such as those involving data integration. There are many scenarios where it is recommended that information be stored in XML format rather than to store it in tables and then compose XML messages from the information. For more information about these scenarios, see the MSDN article XML Best Practices for Microsoft SQL Server 2005. The application of XML for storing documents and for representing semi-structured data has led to the evolution of XML as a data storage format that simplifies data management at the server.

This evolution, however, posed a problem—extracting information from the XML data stored in relational tables as BLOBs required a query language to extract and shape information represented in the XML. Microsoft SQL Server 2000 provided OpenXML which could be used for querying, but was designed for mapping XML data to relational form and thus could not provide full support for the XML data model (see Table 1).

The relational data model and the XML data model differ in a lot of ways. The following table outlines the major differences between the two data models.

Table 1   Differences between the relational and XML data models

FeatureRelational Data ModelXML Data Model
Flat structured dataUses flat tables to store data in a column form. Recommended way to store flat structured data.Useful when it is required to preserve the document order or when the schema is flexible or unknown.
Semi-structured dataIt is difficult to model semi-structured data using relational model.Provides excellent support for representing semi-structured data with variable or evolving schema.
Markup dataNot suitable for storing markup data beyond BLOB storage.Excellent for storing markup data such as HTML, RTF and so on.
Nested or hierarchical data structuresSupports nested data by using multiple tables and linking them with foreign keys, but the complexity of queries required for searching nested data stored in relational form increases when the depth of nesting increases or is unknown.Provides excellent support for expressing nested and hierarchical data.
Order of dataNot preserved.Preserved.
Input dataHomogeneous.Heterogeneous.
Result setHomogeneous.Heterogeneous.

Both the increasing need to deal with more heterogeneously structured data and the necessity of having an implied order are two of the most important reasons why the relational data model is being extended to store XML documents natively. In addition, the limitations of the SQL language in handling semi-structured or markup information resulted in the development of the XQuery language. The XQuery language has been designed from scratch taking into consideration the nature of XML data and the issues involved in processing it.

SQL Server 2005 has built-in support for the native storage of XML data using the XML data type. XQuery 1.0 is a language that is being defined by the World Wide Web Consortium (W3C) XML Query Working Group to formulate queries over XML data. XQuery, like SQL, is a declarative query language that, as we will see later, can be easily understood with a basic knowledge of SQL and XPath.

This paper is based on the implementation of XQuery 1.0 in SQL Server 2005, which in turn is based on the XQuery 1.0 July 2004 working draft. The first section of this paper provides an overview of the new XML data type and its associated features. Subsequent sections introduce the new XQuery language and its advantages, the FLWOR statement, various operators in XQuery, built-in functions in XQuery, type-related expressions, and non-supported features in SQL Server 2005, respectively. Finally, the last sections provide information on best practices and guidelines, XML data modification, and XQuery usage scenarios.

The XML Data Type in SQL Server 2005

The new XML data type introduced in SQL Server 2005 provides users with the ability to store XML documents and fragments in the database. An XML data type can be used to create a column, a parameter to a stored procedure or function, or a variable.

Additionally, the user can associate a column of type XML with an XML schema collection to create a typed XML column. The XML schemas in the collection are used to validate and type the XML instances.

Typed vs. Untyped XML Data Type

An XML data type can be associated with an XML schema collection to have schema constraints enforced on XML instances. If the XML data is associated with an XML schema collection, it is called typed XML. Otherwise, it is called untyped XML.

The SQL Server 2005 XML data type implements the ISO SQL-2003 standard XML data type. It can store not only well-formed XML 1.0 documents, but also so-called XML content fragments with top-level text nodes and an arbitrary number of top-level elements. Checks for well-formedness of the data are performed, and these checks do not require the XML data type to be bound to XML schemas. Data that is not well-formed (subject to the SQL-2003 content relaxations) is rejected.

Untyped XML is useful when the schema is not known a priori. It is also useful when the schema is known but changing rapidly and thus hard to maintain, or multiple schemas exist and are late bound to the data based on external requirements. Furthermore, untyped XML is useful when the XML schemas contain XML Schema constructs not supported by the database engine (e.g. key/keyref, lax validation). In that case, you could use the System.XML validator inside a CLR (common language runtime) user-defined function to provide validation.

If you have XML schemas in an XML schema collection describing your XML data, you can associate the XML schema collection with the XML column to yield typed XML. The XML schemas are used to validate the data, perform more precise type checks than untyped XML during compilation of query and data modification statements, and optimize storage and query processing.

Typed-XML columns, parameters, and variables can store XML documents or content fragments, which you can specify as an option (DOCUMENT or CONTENT, respectively, with CONTENT as the default) at the time of declaration. Furthermore, you have to provide the collection of XML schemas. Specify DOCUMENT if each XML instance has exactly one top-level element; otherwise, use CONTENT. The XQuery compiler uses the DOCUMENT flag information to infer Singleton top-level elements during static type inference.

Methods of the XML Data Type

The XML data type supports five methods that can be used to manipulate XML instances. The methods of the XML data type can be described as follows:

The query() method takes an XQuery expression that evaluates to a list of XML nodes and allows the user to extract fragments of an XML document. The result of this method is an instance of untyped XML.

The value() method is useful for extracting scalar values from XML documents as a relational value. This method takes an XQuery expression that identifies a single node and the desired SQL type to be returned. The value of the XML node is returned cast to the specified SQL type.

The exist() method allows the user to perform checks on XML documents to determine if the result of an XQuery expression is empty or nonempty. The result of this method is 1 if the XQuery expression returns a nonempty result, 0 if the result is empty, and NULL if the XML instance itself is NULL.

Decomposing an XML document into relational data is facilitated by the nodes() method of the XML data type. The nodes() method accepts an XQuery expression and returns a rowset in which each row represents a context node identified by the query expression. Methods of the XML data type such as query(), value(), exist(), and nodes() can also be invoked on the context nodes returned from the nodes() method.

The modify() method can be used to modify the content of an XML document. It accepts XML DML statements to insert, update, or delete one or more nodes from an XML instance. It raises an error if applied to a NULL value.

For more information, see the XML Support in Microsoft SQL Server 2005 white paper.

Introduction to XQuery

XQuery is a new language for querying XML data that allows navigational access based on XPath 2.0. This section provides an overview of various aspects of the XQuery language such as the relationship between XQuery and XPath, advantages of using XQuery, application areas of XQuery, the role of XML Schema in XQuery, and so on.

Overview of XPath 2.0

XPath 1.0, as defined by the W3C, is a language for locating parts of an XML document. XPath uses path-based syntax for identifying nodes in the XML document. It also defines the core syntax for both XSLT 1.0 and XPointer. XPath 1.0 has built-in functions to handle strings, Boolean values, and floating point numbers. It defines the syntax for filtering the node set with the ability to specify filtering criteria. XPath 1.0 is being extended in XPath 2.0 to support a more detailed type system and to provide more functionality. XQuery 1.0 in turn is based on XPath 2.0 and adds ordering, reshaping, construction, and validation capabilities to the navigational and filtering aspects of XPath 2.0.

Overview of XQuery

XQuery is a declarative, typed, functional language designed from scratch by the XML Query Working Group specifically for the purpose of querying data stored in XML format. XQuery shares the same data model and the same XML Schema-based type system with other members of the XML standards family such as XPath 2.0 and XSLT 2.0. XQuery is designed to work with XML documents that are untyped (no schema associated with the data), typed with XML schemas, or a combination of both. As previously mentioned, XQuery 1.0 is basically a superset of XPath 2.0. In addition to the features of XPath 2.0, it has the following capabilities:

  • Adds an order by clause to the FLWOR clause to sort in non-document order.
  • Adds a let clause to the FLWOR clause to name results of expressions for further use (not supported in SQL Server 2005).
  • Provides a way to specify static context items in the query prolog (such as namespace prefix bindings).
  • Provides the ability to construct new nodes.
  • Provides the ability to define user-defined functions (not supported in SQL Server 2005).
  • Provides the ability to create modules/libraries (not supported in SQL Server 2005).

Advantages of XQuery

  • It is easy to learn if knowledge of SQL and XPath is present.
  • When queries are written in XQuery, they require less code as compared to queries written in XSLT.
  • XQuery can be used as a strongly typed language when the XML data is typed, which can improve the performance of the query by avoiding implicit type casts and provide type assurances that can be used when performing query optimization.
  • XQuery can be used as a weakly typed language for untyped data to provide high usability. SQL Server 2005 implements static type inferencing with support for both strong and weak type relationships.
  • Because XQuery requires less code to perform a query than does XSLT, maintenance costs are lower.
  • XQuery is going to be a W3C recommendation and will be supported by major database vendors.

Caveat regarding XQuery 1.0 language as of the writing of this document:

  • The XQuery specification is currently under development and may change in the future. SQL Server 2005 implements a stable part of the W3C working draft.

Application Areas of XQuery

Application areas of XQuery can be classified broadly as follows:

  • XQuery for query/analysis: XQuery is excellent for querying huge chunks of data and provides the capability to filter, sort, order, and repurpose the required information. Typical applications include querying XML documents that represent semi-structured information, name-value pair property bags, analysis of application logs, transaction logs and audit logs to identify potential application errors and security issues, and so on.
  • XQuery for application integration: As organizations move away from proprietary application integration approaches and start adopting standards based application integration approaches, the need for transforming data from internal application-specific formats to standard exchange formats is gaining more focus. Because of its ability to construct and transform XML data, XQuery caters to this need. One typical use of XQuery in the application-integration domain is translating the vocabulary used by one application that uses native XML database/relational data source into a language used by another application that uses XML/relational data format.

Advantages of Using XQuery at the Server

Performing XML processing at the server using XQuery has many advantages compared to client-side XML processing. Some of these advantages can be summarized as follows:

  • Reduced traffic on the network: When XML data is processed on the server, only the results are forwarded to the client. This results in reduced traffic on the network.
  • More security: Only the data that is required is sent to the client thereby avoiding the risk of exposing the entire data to the network as is the case when using client-side XML processing.
  • Better maintainability: Processing XML on the server results in browser independent code on the client, which leads to better maintainability on the client side.
  • Improved performance: Queries written using XQuery on the server are subjected to optimization by the SQL query engine. This results in improved performance when compared to retrieving the entire data and filtering the data at the client. Furthermore, indexes can be created on the XML data type column to achieve enhanced performance.

How XQuery Implementation Uses XML Schemas

The XML schema collection associated with an XML data type is used by the relational engine as follows:

  • To validate the XML instances during insertion operations.
  • To validate the XML instances during modification operations.
  • Type information contained in the XML schema is used during static type checking for early error detection and for query performance improvements by generating better query plans and avoiding many runtime inspections.
  • Type information present in the XML schema is used by the SQL Server to optimize storage.

Structure of the XQuery Expression

An XQuery expression in SQL Server 2005 consists of two sections—a prolog and a body. A prolog can in turn contain a namespace declaration subsection. Namespace declarations are used to define a mapping between prefix and namespace URI thereby enabling you to use the prefix instead of the namespace URI in the query body. You can also refer to element names without the prefix by binding a default namespace for element names, using the declare default namespace declaration.

The body of an XQuery expression contains query expressions that define the result of the query. It can for example be the signature FLWOR expression (see The FLWOR Statement in this paper), an XPath 2.0 expression (see XPath 2.0 Expressions in this paper), or another XQuery expression such as a construction or arithmetic expression.

Example: Specifying default namespace in prolog section of XQuery

The following query selects all Employment nodes for a candidate whose JobCandidateID is 3. The query defines a default namespace and does not use a namespace prefix:

SELECT Resume.query('
   declare default namespace  "http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   /Resume/Employment
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Example: Specifying namespace using the "WITH XMLNAMESPACES" clause

SQL Server also supports the SQL-2003 standard extension that allows a user to declare XML namespace bindings in the SQL WITH clause on a per SQL query basis, thus avoiding repetitive declarations in multiple XML data type method invocations. The following query shows the modified version of the query shown in the previous example. This query declares a namespace using the WITH XMLNAMESPACES clause:

WITH XMLNAMESPACES( 'http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume' AS  "RES")
SELECT Resume.query('
   /RES:Resume/RES:Employment
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

XPath 2.0 Expressions

XQuery uses XPath 2.0 expressions to locate nodes in a document and to navigate from one location to another within a single document or across documents. Navigation paths defined using XPath consist of a sequence of steps separated by /. A single step comprises an axis, a node test, and zero or more step qualifiers.

The axis specifies the direction of movement, relative to the context node. Supported axes in SQL Server 2005 are child, descendant, parent, attribute, self and descendant-or-self.

A node test specifies a condition that all the nodes that are selected by a step must satisfy. The node condition can be specified based on node name or node type.

Step qualifiers can be defined by using predicates or dereferences. A predicate is an expression that acts as a filter on a node sequence and is specified within square brackets. A dereference maps the elements and/or attributes nodes in a node sequence to the nodes that they reference. A node sequence passed as an input to the dereference must contain elements or attributes of type IDREF or IDREFS. The dereference generates a new sequence consisting of the element nodes whose ID-type attribute values match the IDREF values extracted from the elements and attributes in the input sequence.

The steps of an XPath expression are evaluated from left to right. Execution of a step sets the evaluation context items for the next step. A context item in a path expression is a node that is selected as a result of the execution of a step in an XPath expression. A step is evaluated relative to the context item that was obtained in the previous step. The result of an XPath expression is a sequence of nodes in document order that are obtained after executing all the steps in the expression in the order from left to right in the path expression.

This example expression uses the column Resume which is of type XML in table [HumanResources].[JobCandidate] from the AdventureWorks database for the purpose of illustrating the concept of path expressions. The following path expression selects all address nodes for which the address type is set to Home.

//child::ns:Addr.Type[.="Home"]/parent::node()

In the preceding path expression,

  • child is the axis specifier.
  • :: is the axis separator.
  • ns is the namespace prefix.
  • Addr.Type is the node test.
  • [.="Home"] is the predicate expression where . refers to the context node.

XQuery also supports abbreviated syntax for specifying the axis. The following table shows the axis and corresponding abbreviated syntax.

Table 2   Abbreviated syntax for axes

AxisAbbreviated form
Attribute@
Child 
descendant-or-self::node()//
parent::node()..
self::node().

Example: Selecting organization names from employment history

The following XPath expression selects the children text nodes of Emp.OrgName elements that are child nodes of node Resume/Employment. Here, text() is used to select the text node of the Emp.OrgName element :

/Resume/Employment/Emp.OrgName/text()

The FLWOR Statement

FLWOR statements form the core expressions of XQuery and are similar to the SELECT statements of SQL. The acronym FLWOR (pronounced "flower") stands for FOR, LET, WHERE, ORDER BY, RETURN. FLWOR expressions in XQuery enable users to specify operations such as declarative iteration, variable binding, filtering, sorting, and returning the results. SQL Server 2005 supports FOR, WHERE, ORDER BY, and RETURN.

For

The for clause in a FLWOR expression enables users to define a declarative iteration of a bound variable over an input sequence. The input sequence can be specified using XPath expressions, sequence of atomic values, a sequence constructed using literals, or constructor functions. It is therefore analogous to the SQL SELECT FROM clause and is not like a programming language "for" construct.

Variable binding is also specified in the for clause.

Example: Selecting all home address elements from resume using the for clause

The following query selects all Address nodes where the type of address is set to Home and the JobCandidateID is 3:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $A in /RES:Resume/RES:Address/RES:Addr.Type[.="Home"]/..
   return 
   $A 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

where

The where clause filters the results of an iteration by applying the expression specified with the where clause.

Example: Selecting all home address elements using the where clause

The following query selects all Address nodes where the type of address is set to Home and the JobCandidateID is 3:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $A in /RES:Resume/RES:Address
   where $A/RES:Addr.Type[.="Home"]
   return 
   $A 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

order by

The order by keyword enables you to sort the values in the returned result set. The order by keyword accepts a sorting expression, which should return an atomic value. Optionally, you can also specify ascending or descending for the sort order. The default sort order is ascending.

Example: Selecting employment history in ascending order using the order by clause

The following query selects all Employment nodes in ascending order of employment starting date for a candidate whose JobCandidateID is 3:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $EMP in /RES:Resume/RES:Employment
   order by $EMP/RES:Emp.StartDate
   return 
   $EMP 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

return

The return clause, which is analogous to the SELECT clause in SQL, enables users to define the result of a query. You can specify any valid XQuery expression in the return clause. You can also construct XML structures in the return section by specifying constructors for elements, attributes, etc.

Example: Selecting specific elements of employment history using the return clause

The following query selects StartDate, EndDate, OrgName, JobTitle elements of Employment node for a candidate whose JobCandidateID is 3:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $EMP in /RES:Resume/RES:Employment
   order by $EMP/RES:Emp.StartDate
   return
     <Employment>
      { $EMP/RES:Emp.StartDate }
      { $EMP/RES:Emp.EndDate }
      { $EMP/RES:Emp.OrgName }
      { $EMP/RES:Emp.JobTitle }
     </Employment>
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Current implementation of XQuery in SQL Server 2005 does not support the let clause. This aspect is discussed further in Non-supported Features and Workarounds in this paper.

FLWOR Expressions vs. XPath Expressions

Using a FLWOR expression to define the result sequence when the sequence can be expressed using an XPath expression incurs a performance penalty because the query plan contains a JOIN operation between the for variable and the body of the for clause. The use of a FLWOR expression is justified only if one or more of the following conditions are satisfied:

  • If you want to iterate over a sequence of values that are returned as a result of an expression. This is achieved using the for clause, which binds a variable to successive values of the result set. Examples are the construction of new elements within the scope of the for clause and the retention of duplicates.
  • When you want to filter the result sequence of the for clause based on a predicate which cannot be defined using simple XPath expressions. The where clause is used to eliminate unwanted values in the result set. An example is as follows:
    DECLARE @Result xml
    SET @Result = '<Result />'
    SELECT @Result.query('
    for $i in (1, 2, 3), $j in (3, 4, 5)
    where $i < $j
    return sum($i + $j)
    ') as Result
    
    
  • If you want to sort the result set based on a sorting expression. Sorting is defined on the result set using the order by clause.
  • When you want to define the shape of the returned result set using the results obtained from the for clause. The return statement is used to perform the shaping of the result set.

In all other cases, using XPath expressions is recommended.

Operators in XQuery

As a functional language, XQuery in SQL Server 2005 supports various types of functions and operators that can be grouped under the following categories:

  • Arithmetic operators
  • Comparison operators
  • Logical operators

Table 3   Operators supported in SQL Server 2005

TypeOperators
Arithmetic operators+,-,*,div, mod
General comparison operators=, !=, <, >, <=, >=
Value comparison operatorseq, ne, lt, gt, le, ge
Node comparison operatoris
Node order comparison operators>>, <<
Logical operatorsand, or

Arithmetic Operators

SQL Server 2005 supports five arithmetic operators. These are +, b, *, div, and mod. Currently, it does not support idiv.

Example: Converting selected values of store survey information

This example is based on the [Sales].[Store] table in the AdventureWorks database. The following query returns values of AnnualSales, AnnualRevenue in yen and the store area in square meters for a store whose CustomerID is 3:

SELECT Demographics.query('
   declare namespace ST="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/StoreSurvey";
   for $S in /ST:StoreSurvey
   return
     <StoreDetails
        SalesInYen ="{ $S/ST:AnnualSales*106.8100  }"
        RevenueInYen = "{ $S/ST:AnnualRevenue*106.8100  }"
        StoreAreaInSqMeters = "{ $S/ST:SquareFeet*0.0929  }">
     </StoreDetails>
') as Result
FROM [Sales].[Store]
WHERE CustomerID = 3

Comparison Operators

SQL Server 2005 has implemented support for four types of comparison operators—general comparison operators, value comparison operators, node comparison operators, and node order comparison operators.

General comparison operators

General comparison operators help compare atomic values, sequences, or a combination of the two. General comparison operators are =, !=, <, >, <=, and >=. A general comparison is existentially quantified, meaning that any match will result in true.

Example: Selecting all address elements where the address type is not set to Home

The following query selects all Address nodes where the type of address is not set to Home and the JobCandidateID is 3:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $A in /RES:Resume/RES:Address
   where $A/RES:Addr.Type[.!="Home"]
   return 
   $A 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Value comparison operators

Value comparison operators help compare atomic values. Value comparison operators supported by SQL Server 2005 are eq, ne, lt, gt, le, and ge. The current implementation of XQuery with respect to promoting untyped atomic values is not aligned with the July 2004 draft of the XQuery specification. The untyped atomic type is promoted to the type of the other operand instead of xs:string as specified in the XQuery specification. This is to maintain consistency across general and value comparison operators which we consider more important than making the value comparison transitive.

Example: Selecting all education elements where the GPA is greater than 3.5

The following query selects all Education nodes where the GPA is greater than 3.5 for the candidate whose JobCandidateID is 2:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $ED in /RES:Resume/RES:Education
   where xs:decimal($ED/RES:Edu.GPA) gt 3.5 
   return 
   $ED 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 2

Node comparison operators

You can use the node comparison operator is to compare two nodes to determine if they represent the same node or not. The node comparison operator accepts two operands that are of type node.

Example: Comparing two address nodes to check their identity

The following query compares two address nodes to check if they represent the same node in the document:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   if ( (/RES:Resume/RES:Address)[1] is (//RES:Address)[1] )
   then
      <Result>Nodes are equal</Result> 
   else
      <Result>Nodes are not equal</Result> 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Node order comparison operators

You can use the node order comparison operators to ascertain the order of two nodes in a document. The node order comparison operators supported by SQL Sever 2005 are >> and << and both the operators accept two operands. The >> operator returns true if the left operand precedes the right operand in document order, and the << operator returns true if the left operand follows the right operand in document order.

Example: Comparing the order of two address nodes

The following query compares the order of two address nodes for a candidate whose JobCandidateID is 3:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   if ( (/RES:Resume/RES:Address/RES:Addr.Type[.="Home"])[1] << (/RES:Resume/RES:Address/RES:Addr.Type[.="Permanent"])[1] )
   then
      <Result>Home address precedes Permanent address</Result> 
   else
      <Result>Home address follows Permanent address</Result> 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Logical Operators

The logical operators supported by XQuery in SQL Server 2005 are and and or. The value of any logical expression formed using these operators can be either true or false.

Example: Using the and operator to create a logical expression

The following query returns the candidate's education element that contains bachelor level business degree:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   /RES:Resume/RES:Education[RES:Edu.Level="Bachelor" and RES:Edu.Major="Business"]
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

The if-then-else Construct

Like other functional languages, XQuery supports the if-then-else construct. You can use the if-then-else statement to perform operations based on the value of a conditional expression.

Example: Using a conditional expression

The following query displays the type of address that is specified in the resume for a candidate whose JobCandidateID is 3:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $A in /RES:Resume/RES:Address
   return 
      if ( $A/RES:Addr.Type eq "Home" )
      then
         <Result>Home Address</Result> 
      else
         <Result>Other Address</Result> 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Constructing XML Using XQuery

XQuery constructors enable you to create XML structures within a query. Constructors are available for elements, attributes, processing instructions, text nodes, and comments.

The following examples illustrate these approaches to constructing XML.

Example: Using constant expressions

The following query displays employment history details constructed using constant expressions:

SELECT Resume.query('
     <Employer IndustryCategory="ITServices">
      <Organization>ABC Technologies</Organization>
      <JobTitle>Software Engineer</JobTitle>
      <StartDate>2001-10-01</StartDate>
      <EndDate>2003-05-09</EndDate>
     </Employer>
') as Result
FROM [HumanResources].[JobCandidate]

WHERE JobCandidateID = 3

Example: Using data obtained dynamically

The following query displays employment history details constructed using the results obtained from a query for a candidate whose JobCandidateID is 3:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $EMP in /RES:Resume/RES:Employment
   return
     <Employer Organization = "{ $EMP/RES:Emp.OrgName }" >
      { $EMP/RES:Emp.StartDate }
      { $EMP/RES:Emp.EndDate }
      { $EMP/RES:Emp.JobTitle }
     </Employer>
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Example: Using constant names for computed element and attribute constructors

The following query displays the employment history of a candidate whose JobCandidateID is 3:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $EMP in /RES:Resume/RES:Employment
   return
     element Employer
     {
      attribute Organization { $EMP/RES:Emp.OrgName },
      element StartDate { string($EMP/RES:Emp.StartDate) },
      element EndDate { string($EMP/RES:Emp.EndDate) },
      element JobTitle { string($EMP/RES:Emp.JobTitle) }
     }
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Performing XQuery Construction vs. Shaping Using FOR XML

Some applications have a requirement to generate XML from a rowset. On the server, XML can be generated using a FOR XML clause or XQuery constructions or XML DML operations. Recommendations on using FOR XML and XQuery constructors for constructing XML are as follows:

  • If XML is to be aggregated from multiple columns and multiple rows, then FOR XML is the only choice.
  • If a single XML instance is to be reshaped, then it can be done using XQuery as well as FOR XML. XQuery might be faster since the FOR XML approach will require multiple invocations of XML data type methods on the XML instance.
  • You can use multiple XML DML statements to construct an XML instance. This approach is significantly slower than XQuery construction.
  • Use the new TYPE directive available with the FOR XML clause in SQL Server 2005 to generate the result of a FOR XML query as an instance of XML data type.

Built-in XQuery Functions

Implementation of XQuery in SQL Server 2005 supports a subset of the built-in functions of XQuery 1.0 and XPath 2.0. These functions include data accessor functions, string manipulation functions, aggregate functions, context functions, numeric functions, Boolean functions, node functions, and sequence functions. The following sections explore some of these functions.

Data Accessors

You can use the data accessor functions to extract values of nodes as strings or typed values. XQuery supports two types of data accessor functions: string(), which extracts the string value of an item and data(), which gets the typed value. If the node is not a text node, an attribute node, or an element node then the data() function throws a static error. If the node is a document node of an untyped XML instance, then data() returns a string value of the document. The data() function returns a static error if the node is a complex typed element.

Example: Using the string() function

See the Using constant names for computed element and attribute constructors example in the section "Constructing XML Using XQuery" in this paper for a query that generates the employment history of a candidate by using the string() function and computed element constructors.

Example: Using the data() function

The following query generates the employment history of a candidate by using the data() function and computed element constructors:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $ED in /RES:Resume/RES:Education
   where xs:decimal( data($ED/RES:Edu.GPA) ) gt 3.5 
   return 
     element Education
     {
      element Level { data($ED/RES:Edu.Level) },
      element Degree { data($ED/RES:Edu.Degree) },
      element GPA { data($ED/RES:Edu.GPA) },
      element GPAScale { data($ED/RES:Edu.GPAScale) }
     }
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 2

String Manipulation

XQuery supports four string manipulation functions:

  • concat() Helps concatenate two or more strings.
  • contains() Helps determine whether or not a string specified as the first operand contains another string specified as the second operand. The length of the search string is limited to 4,000 Unicode characters.
  • substring() Helps extract portion of a string from another string known as source string.
  • string-length() Helps calculate the length of a string.

The current release of SQL Server 2005 supports only the Unicode codepoint collation.

Example: Using the concat() and substring() functions

The following query generates the employment history of a candidate by concatenating the values of start date, end date, organization name, and job title:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $EMP in /RES:Resume/RES:Employment
   return
     <Employment>
     {
      concat( substring(string($EMP/RES:Emp.StartDate),1,10)," to ",
        substring(string($EMP/RES:Emp.EndDate),1,10), ", ",
        string($EMP/RES:Emp.OrgName), ", ",
        string($EMP/RES:Emp.JobTitle) )
     }
     </Employment>
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Example: Using the contains() function

The following query demonstrates the use of the contains() function by displaying Education details for a node that contains the string science in the value of the element Edu.Degree:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $ED in /RES:Resume/RES:Education
   where contains($ED/RES:Edu.Degree, "Science")
   return
     element Education
     {
      element Level { data($ED/RES:Edu.Level) },
      element Degree { data($ED/RES:Edu.Degree) },
      element GPA { data($ED/RES:Edu.GPA) },
      element GPAScale { data($ED/RES:Edu.GPAScale) }
     }
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 2

Aggregate Functions

Aggregate functions operate on a sequence of items and return the aggregate values of the sequence. Aggregate functions currently supported in the XQuery support in SQL Server 2005 are count(), min(), max(), avg(), and sum(). The functions min() and max() accept only base types that support the gt operator (i.e., the three built-in numeric base types, the date/time base types, xs:string, xs:boolean, and xdt:untypedAtomic). A sequence of mixed types is not supported in these functions. Furthermore, xdt:untypedAtomic is treated as xs:double.

For avg() and sum(), the type of the passed expression needs to be a subtype of one of the three built-in numeric base types or untypedAtomic (but not a mixture, xdt:untypedAtomic is treated as xs:double).

The count() function returns the number of items in a sequence.

Example: Using the count() function

The following query displays the count of employment, education, and address elements present in the document using the count() function:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   <Employment>Element count is { count(/RES:Resume/RES:Address) }</Employment>,
   <Education>Element count is { count(/RES:Resume/RES:Education) }</Education>,
   <Address>Element count is { count(/RES:Resume/RES:Address) }</Address>
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Example: Using the min() function

The following query displays the education element for which the GPA value is minimum using the min() function:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $ED in /RES:Resume/RES:Education
   where $ED/RES:Edu.GPA = min(/RES:Resume/RES:Education/RES:Edu.GPA)
   return 
   $ED 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 2

Example: Using the max() function

The following query displays the education element for which the GPA value is maximum using the max() function:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $ED in /RES:Resume/RES:Education
   where $ED/RES:Edu.GPA = max(/RES:Resume/RES:Education/RES:Edu.GPA)
   return 
   $ED 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 2

Example: Using the avg() function

The following query calculates weekly average high and low temperatures for the cities of New York and Boston using the avg() function:

DECLARE @Weather xml
SET @Weather = '
<WeatherInfo>
   <NewYork>
      <Temp Date="2004-11-01" High="55" Low="45" />
      <Temp Date="2004-11-02" High="58" Low="42" />
      <Temp Date="2004-11-03" High="60" Low="40" />
      <Temp Date="2004-11-04" High="51" Low="47" />
      <Temp Date="2004-11-05" High="54" Low="41" />
      <Temp Date="2004-11-06" High="55" Low="43" />
      <Temp Date="2004-11-07" High="58" Low="47" />
   </NewYork>
   <Boston>
      <Temp Date="2004-11-01" High="53" Low="45" />
      <Temp Date="2004-11-02" High="56" Low="42" />
      <Temp Date="2004-11-03" High="54" Low="41" />
      <Temp Date="2004-11-04" High="52" Low="45" />
      <Temp Date="2004-11-05" High="52" Low="36" />
      <Temp Date="2004-11-06" High="54" Low="41" />
      <Temp Date="2004-11-07" High="56" Low="44" />
   </Boston>
</WeatherInfo>'

SELECT @Weather.query('
   <WeatherInfo>
      <NewYork>
         <AvgHigh>{ avg(/WeatherInfo/NewYork/Temp/@High) }</AvgHigh>
         <AvgLow>{ avg(/WeatherInfo/NewYork/Temp/@Low) }</AvgLow>
      </NewYork>
      <Boston>
         <AvgHigh>{ avg(/WeatherInfo/Boston/Temp/@High) }</AvgHigh>
         <AvgLow>{ avg(/WeatherInfo/Boston/Temp/@Low) }</AvgLow>
      </Boston>
   </WeatherInfo>
') as Result

Example: Using the sum() function

The following query calculates weekly average high and low temperatures for the cities of New York and Boston using the sum() and count() functions:

DECLARE @Weather xml
SET @Weather = '
<WeatherInfo>
   <NewYork>
      <Temp Date="2004-11-01" High="55" Low="45" />
      <Temp Date="2004-11-02" High="58" Low="42" />
      <Temp Date="2004-11-03" High="60" Low="40" />
      <Temp Date="2004-11-04" High="51" Low="47" />
      <Temp Date="2004-11-05" High="54" Low="41" />
      <Temp Date="2004-11-06" High="55" Low="43" />
      <Temp Date="2004-11-07" High="58" Low="47" />
   </NewYork>
   <Boston>
      <Temp Date="2004-11-01" High="53" Low="45" />
      <Temp Date="2004-11-02" High="56" Low="42" />
      <Temp Date="2004-11-03" High="54" Low="41" />
      <Temp Date="2004-11-04" High="52" Low="45" />
      <Temp Date="2004-11-05" High="52" Low="36" />
      <Temp Date="2004-11-06" High="54" Low="41" />
      <Temp Date="2004-11-07" High="56" Low="44" />
   </Boston>
</WeatherInfo>'

SELECT @Weather.query('
   <WeatherInfo>
      <NewYork>
         <AvgHigh>{ sum(/WeatherInfo/NewYork/Temp/@High) div count(/WeatherInfo/NewYork/Temp/@High) }</AvgHigh>
         <AvgLow>{ sum(/WeatherInfo/NewYork/Temp/@Low) div count(/WeatherInfo/NewYork/Temp/@Low) }</AvgLow>
      </NewYork>
      <Boston>
         <AvgHigh>{ sum(/WeatherInfo/Boston/Temp/@High) div count(/WeatherInfo/Boston/Temp/@High) }</AvgHigh>
         <AvgLow>{ sum(/WeatherInfo/Boston/Temp/@Low) div count(/WeatherInfo/Boston/Temp/@Low) }</AvgLow>
      </Boston>
   </WeatherInfo>
') as Result

Context Functions

You can use the context functions to obtain the contextual properties of a context item. SQL Server 2005 implements two context functions—last() and position(). The last() function can be used to determine the number of items in a sequence and the position() function can be used to obtain the position of a context item. Both the last() and position() functions without an argument can only be used in the context of a context-dependent predicate (i.e., inside []) in SQL Server 2005.

Example: Using the last() function

The following query retrieves the last address element for a candidate using the last() function:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   /RES:Resume/RES:Address[last()]
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Example: Using the position() function

The following query retrieves the first two address elements for a candidate using the position() function:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   /RES:Resume/RES:Address[position()<=2]
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Type-Related Expressions

XQuery supports various types of expressions or operators that are based on the type information. These expressions can be classified as type assertion expressions, type inspection expressions, and type casting expressions. These expressions are discussed briefly in the following sections.

Type Assertion Expressions

as xs:TYPE clause in for statement

You can use the as clause to specify the type for the binding variable used in the for statement.

When a type is declared for the binding variable, binding values that are not of the declared type would result in type error. The xs:TYPE clause is not a cast expression but it serves as a type assertion.

Example: Using "as xs:TYPE" clause with for statement

The following query binds the address node sequence to a variable $A which is defined as type element(RES:Address):

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $A as element(RES:Address) in /RES:Resume/RES:Address
   return   
      $A
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 2

Type Inspection Expressions

instance of xs:TYPE operator

The instance of operator helps identify the runtime type of an item in an XML document.

Example: Using "instance of xs:TYPE" operator

The following query checks to see if the type of an address node identified by an XPath expression matches element() type or not:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   if ( (/RES:Resume/RES:Address)[1] instance of element() )
   then
      <Result>Selected node is an Element</Result> 
   else
      <Result>Selected node is not an Element</Result> 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Type Casting Expressions

Implicit Type Casting

The XQuery engine performs implicit type casting for numeric types and untypedAtomic values in expressions that contain arithmetic operations or function invocations. This process is known as type promotion. Type promotion occurs when an expression results in a numeric type that is incompatible with the expected numeric type. Type promotion is performed by casting the resulting expression to the required type.

Example: Implicit type casting

The following query performs an arithmetic operation on a decimal value and a double value. In the current scenario, the values in the expression are added only after promoting the xs:decimal value to xs:double.

DECLARE @Result xml
SET @Result = '<Result />'
SELECT @Result.query('
   <Result>{ xs:decimal("10.55") + xs:double(1.5e1) }</Result>
') as Result

Explicit Type Casting

Typed value constructors

XQuery provides constructor functions for all built-in types defined in the XML Schema specification. These constructors are useful for constructing typed values and also for casting values from one type to another. XQuery also makes constructors available for types that are defined in imported schemas.

Example: Using a value constructor for constructing values

The following query returns all Employment nodes for which the StartDate is greater than a value constructed using constructor for type xs:date:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $EMP in /RES:Resume/RES:Employment
   where $EMP/RES:Emp.StartDate gt xs:date("1995-01-01")
   return   
      $EMP
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 3

Example: Using a value constructor for typecasting

The following query selects all Education nodes where the GPA is greater than or equal to 3.8 for the candidate whose JobCandidateID is 2. This query uses the value constructor for xs:decimal for typecasting the value of Edu.GPA from xs:string to xs:decimal:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $ED in /RES:Resume/RES:Education
   where xs:decimal( data($ED/RES:Edu.GPA) ) ge 3.8 
   return
     element Education
     {
      element Level { string($ED/RES:Edu.Level)},
      element StartDate { string($ED/RES:Edu.StartDate)},
      element EndDate { string($ED/RES:Edu.EndDate)},
      element Degree { string($ED/RES:Edu.Degree)},
      element GPA { string($ED/RES:Edu.GPA)},
      element GPAScale { string($ED/RES:Edu.GPAScale)}
     }
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 2

cast as xs:TYPE ? Operator

XQuery in SQL Server 2005 supports the cast as TYPE ? operator, which is useful for performing explicit type casting. Explicit type casting can also be performed using the xs:TYPE() constructors, which are more convenient to write than the cast as TYPE ? operator.

Example: Using "cast as xs:TYPE ?" operator

The following query generates an XML that contains typed values of selected elements from Education node set for a candidate whose JobCandidateID is 3:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $ED in /RES:Resume/RES:Education
   return
     element Education
     {
      element Level { $ED/RES:Edu.Level cast as xs:string? },
      element StartDate { $ED/RES:Edu.StartDate cast as xs:date? },
      element EndDate { $ED/RES:Edu.EndDate cast as xs:date? },
      element Degree { $ED/RES:Edu.Degree cast as xs:string? },
      element GPA { $ED/RES:Edu.GPA cast as xs:decimal? },
      element GPAScale { $ED/RES:Edu.GPAScale cast as xs:decimal? }
     }
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 2

Example: Using "xs:TYPE()" operator

The following query generates the same results as the query in the previous example using the xs:TYPE() operator instead of the cast as xs:TYPE ? operator:

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $ED in /RES:Resume/RES:Education
   return
     element Education
     {
      element Level { xs:string($ED/RES:Edu.Level) },
      element StartDate { xs:date($ED/RES:Edu.StartDate) },
      element EndDate { xs:date($ED/RES:Edu.EndDate) },
      element Degree { xs:string($ED/RES:Edu.Degree) },
      element GPA { xs:decimal($ED/RES:Edu.GPA) },
      element GPAScale { xs:decimal($ED/RES:Edu.GPAScale) }
     }
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = 2

Accessing Relational Columns and Variables

When writing queries using XQuery, it is not uncommon to have the requirement for accessing relational columns and variables from within the query. SQL Server 2005 caters to this requirement by implementing two functions—sql:column() and sql:variable().

The function sql:column() can be used to access non-XML columns in a relational table from within a query. This function is useful for scenarios such as aggregating information from XML and non-XML type columns of one or more tables, using the values of non-XML columns to filter the results of an XQuery, and so on. The functions sql:column() and sql:variable() cannot be used with datetime, CLR user-defined functions or XML .

Example: Using sql:column() function

The following query generates an XML that contains values from the CustomerID and Name columns of non-XML data type and values of YearOpened, NumberOfEmployees, AnnualSales, and AnnualRevenue elements from the Demographics column:

SELECT Demographics.query('
   declare namespace ST="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/StoreSurvey";
   element CustomerInfo
   {
      element CustomerID { sql:column("Store.CustomerID") },
      element Name { sql:column("Store.Name") },
      element YearOpened { string((/ST:StoreSurvey/ST:YearOpened)[1]) },
      element NumberOfEmployees { string((/ST:StoreSurvey/ST:NumberEmployees)[1]) },
      element AnnualSales { string((/ST:StoreSurvey/ST:AnnualSales)[1]) },
      element AnnualRevenue { string((/ST:StoreSurvey/ST:AnnualRevenue)[1]) }
   }
') as Result
FROM [Sales].[Store] Store 
WHERE Store.CustomerID = 4

Example: Using sql:variable() function

The following stored procedure returns home address nodes of a specified candidate by using the value of the @AddrType variable in the XQuery where clause to filter the results of the query:

CREATE PROCEDURE [GetCandidateAddress]
   @JobCandidateID [int],
   @AddrType [varchar](20)
AS
BEGIN
   SET NOCOUNT ON;

SELECT Resume.query('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   for $A in /RES:Resume/RES:Address
   where $A[ RES:Addr.Type = sql:variable("@AddrType") ]
   return 
   $A 
') as Result
FROM [HumanResources].[JobCandidate]
WHERE JobCandidateID = @JobCandidateID

END

Example: Using sql:variable() function in conjunction with the exist() method of the XML data type

The following query returns resumes of candidates who hold a Bachelor's degree with a major in Business:

DECLARE @EducationLevel varchar(20)
SET @EducationLevel = 'Bachelor'

DECLARE @Major varchar(20)
SET @Major = 'Business'

SELECT JobCandidateID, Resume
FROM [HumanResources].[JobCandidate]
WHERE Resume.exist ('
   declare namespace RES="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/Resume";
   /RES:Resume/RES:Education[ RES:Edu.Level = sql:variable("@EducationLevel") and RES:Edu.Major = sql:variable("@Major") ]') = 1

Non-Supported Features and Workarounds

Current implementation of XQuery in SQL Server 2005 does not support the following features:

  • The let clause: The let clause, which is a part of the FLWOR expression is useful for binding a variable to the results of an expression. Workaround—Instead of using the let clause, use an inline expression.
  • Range expression (to operator): A range expression can be used to construct a sequence of consecutive integers using the to operator. For example, using a range expression such as (6 to 10), it is possible to construct the sequence (6, 7, 8, 9, 10). Workaround—Instead of using the to operator, list all items in your sequence.
  • Type information: Some features that are based on type system such as typeswitch, treat as, castable, and validate expressions are currently not supported.
    • The functionality of a typeswitch expression is similar to the switch-case construct available in other programming languages. In a switch-case statement, the branches/cases are selected based on the value of the argument passed to the switch. In a typeswitch expression, the branches are selected based on the type of the argument passed to typeswitch. Workaround—use if then else and instance of.
    • The treat as expression can be used to change the static type of the result of an expression to a specific static type and raises a static type error if the static type of the expression does not match the specified type. It does not change the dynamic type or value of the expression. Workaround—none
    • The castable expression is useful for checking whether an atomic value can be cast to the specified type. Workaround—Instead of using the expression "$x castable as T?", use the expression "empty(data($x)) or not(empty(T($x)))"
    • The validate expression performs validation on its argument based on the schema definitions present in the current scope. Workaround—Instead of using the validate expression, use the Transact-SQL casting to the requested schema collection.
  • Reusability: As in other programming languages, it is possible to write reusable functions, known as user-defined functions, that contain useful and complex queries. In addition, it is also possible to package a collection of user-defined functions as modules. Queries can make use of the functions available in modules by importing the modules. Modules can be imported into a query by including them in the prolog section of the query. No workaround in SQL Server 2005.
  • Built-in functions: The following built-in functions are currently not supported in SQL Server 2005. For more information about these functions, see XQuery 1.0 and XPath 2.0 Functions and Operators on the W3C.org Web site.
    • Accessors: fn:node-name(), fn:nilled(), fn:base-uri(), fn:document-uri().
    • Error function: fn:error().
    • Trace function: fn:trace().
    • Functions on numeric values: abs(), round-half-to-even().
    • String-handling functions: codepoints-to-string(), string-to-codepoints(), compare(), string-join(), normalize-space(), normalize-unicode(), upper-case(), lower-case(), translate(), escape-uri(), starts-with(), ends-with(), substring-before(), substring-after(), matches(), replace(), tokenize().
    • Functions and operators for anyURI: resolve-uri().
    • Functions and operators on durations, dates, and time: years-from-duration(), months-from-duration(), days-from-duration(), hours-from-duration(), minutes-from-duration(), seconds-from-duration(), year-from-dateTime(), month-from-dateTime(), month-from-dateTime(), day-from-dateTime(), hours-from-dateTime(), minutes-from-dateTime(), seconds-from-dateTime(), timezone-from-dateTime(), year-from-date(), month-from-date(), day-from-date(), timezone-from-date(), hours-from-time(), minutes-from-time(), seconds-from-time(), timezone-from-time(), adjust-dateTime-to-timezone(), adjust-date-to-timezone(), adjust-time-to-timezone(), subtract-dateTimes-yielding-yearMonthDuration(), subtract-dateTimes-yielding-dayTimeDuration(), subtract-dates-yielding-yearMonthDuration(), subtract-dates-yielding-dayTimeDuration(). Also types xdt:dayTimeDuration and xdt:yearMonthDuration are not supported.
    • Functions related to QNames: resolve-QName(), QName(), namespace-uri-for-prefix(), in-scope-prefixes().
    • Functions on nodes: name(), lang(), root(). Workaround—use / instead of root().
    • Functions and operators on sequences: fn:boolean(), fn:index-of(), fn:exists(), insert-before(), remove(), reverse(), subsequence(), unordered(), zero-or-one(), one-or-more(), exactly-one(), deep-equal(),two-argument version of id(), idref(), doc(), collection() functions and union, intersect and except operators. Workarounds—use not(not()) instead of fn:boolean(), use not(empty()) instead of fn:exists(). Use either an explicit for iteration or [1] instead of zero-or-one() or exactly-one().
    • Context functions: current-dateTime(), current-date(), current-time(), default-collation(), implicit-timezone().
  • Positional variable: Positional variables can be defined as part of a FLWOR statement using the at clause and are useful for identifying the location of an item in the result of an expression.
  • Order modifier: The order modifier "empty greatest | least" specified with an order by clause is not supported.
  • Other features: The following features are not supported:
    • The idiv operator.
    • Explicit schema import is not supported.
    • External variables are not supported.
    • Boundary white space preservation option is not supported.
    • Concatenation of heterogeneous sequences such as nodes and values is not supported.
    • No support for time zone preservation.
    • Accessing a text node of an element with a simple typed content is not supported.
    • No support for accessing xsi:* attributes in a typed XML data type instance.

Best Practices and Guidelines

  • Take advantage of type information if it is available. This will provide performance improvements and the ability to perform static type checking to detect errors.
  • Use XQuery for property promotions if you want to extract the values of a few properties from XML data type and use them in relational queries. Frequently used properties are promoted to relational columns and indexed for better performance.
  • Use an ordinal, such as [1], or explicit FLWOR to avoid static errors due to cardinality mismatch.
  • Use explicit casts to avoid static type errors.
  • Index usage: Create a PATH index if you have queries with heavy use of XPath expressions. Use a VALUE index when the XQuey query contains XPath expressions that involve searching for element or attribute values with imprecisely known paths (e.g., /a/* or //b). PROPERTY index is useful when the query involves searching for all occurrences of a known property within an XML instance.
  • Use a default namespace when most of the types referred are part of a single namespace. Otherwise, use a prefix.

For more information on best practices, see the MSDN articles XML Best Practices for Microsoft SQL Server 2005 and Performance Optimizations for the XML Data Type.

XML Data Modification

SQL Server 2005 provides support for modifying XML instances stored in the database. The current version of the W3C XQuery working draft does not define a syntax for modifying XML documents. In order to provide a mechanism for modifying XML documents, Microsoft has developed XML Data Modification Language (DML). XML documents can be modified using the modify method of the XML data type and specifying the modification using XML DML statements.

XML DML uses the insert, delete, and replace value of keywords to support insert, delete, and update operations on XML documents. Modification of a typed XML instance is subjected to validation checks according to the schema constraints defined on the XML data type.

The following table and XML instance are used to illustrate XML DML operations:

Table:

CREATE TABLE [CandidateInfoXMLDataType]
(
   [JobCandidateID] [int] IDENTITY(1,1) NOT NULL,
   [Resume] [xml] NOT NULL,
   [ModifiedDate] [datetime] NOT NULL DEFAULT (getdate())
)

Sample XML Instance:

<JobCandidate>    
   <Name>      
      <FirstName>Mike</FirstName>      
      <MiddleName></MiddleName>      
      <LastName>Chen</LastName>    
   </Name>    
   <Address>      
      <Address1>34 181st Place SE</Address1>      
      <Address2>Apt 3344</Address2>      
      <City>Redmond</City>
      <State>WA</State>      
      <Country>US</Country>      
      <PhoneNumber>9870909023</PhoneNumber>    
   </Address>    
   <Education>      
      <BachelorDegree>BS</BachelorDegree>      
      <MasterDegree>MS</MasterDegree>    
   </Education>    
   <Skills>      
      <Skill>ASP.NET</Skill>      
      <Skill>SQL</Skill>      
   </Skills> 
   <Employement>
      <Employer>
         <OrgName>ABC Technologies</OrgName>
            <Location>NY, US</Location>
         <StartDate>20/02/2000</StartDate>
         <EndDate>10/04/2004</EndDate>
         <JobTitle>Project Leader</JobTitle>
         <Responsibility>Responsible for design,development,testing activities</Responsibility>
      </Employer>
   </Employement>
</JobCandidate>

The Insert Operation

You can use the insert keyword to insert one or more nodes in an XML document. The insert keyword accepts an XQuery expression that identifies the nodes to be inserted and another XQuery expression that specifies the reference node.

In addition, you can include keywords such as into, after, and before to specify the position of new nodes in relation to the reference node. When you specify the into keyword, new nodes are inserted as children of the reference node. If you include the into keyword, you must also specify the as first or the as last keyword to indicate the position of inserted nodes with respect to the existing child nodes of the reference node. You can also insert new nodes as sibling nodes after or before the reference node by specifying the after or the before keyword.

If the target expression (Expression2) does not identify a single node statically, the insert operation will fail with a static error.

Syntax:

insert 
   Expression1 (
         {{{as first | as last} into} | after | before}
    Expression2 )

Example: Inserting a skill

The following stored procedure allows the user to insert a new skill for a specified candidate. This stored procedure uses the sql:variable() function to access a Transact-SQL variable inside the XML DML statements. For details on how to bind non-XML relational data inside XML, see Accessing Relational Columns and Variables.

The following stored procedure is written based on the assumption that the user will pass a string value of one skill as the second argument to the stored procedure. This stored procedure can be modified to accept an XML fragment that contains one or more skill elements, thereby enabling the user to insert multiple skill nodes, with a single call, to the stored procedure.

/* Stored procedure to insert a new skill element for a candidate */
CREATE PROCEDURE [InsertSkillInfo]
   @JobCandidateID [int],
   @Skill [varchar](200)
AS
BEGIN
   SET NOCOUNT ON;

   UPDATE [CandidateInfoXMLDataType]
   SET Resume.modify('
   insert element Skill {sql:variable("@Skill")}
   as last
   into (/JobCandidate/Skills)[1]
   ')
   WHERE JobCandidateID = @JobCandidateID
END

The Delete Operation

The delete keyword enables you to delete one or more nodes from an XML instance. The delete keyword accepts an XQuery expression that identifies one or more nodes to be deleted from the XML document.

Syntax:

delete Expression

Example: Deleting a skill

The following example illustrates the use of the delete keyword to delete a skill for a specified candidate.

The following stored procedure is written based on the assumption that the user will pass a string value of one skill as the second argument to the stored procedure. This stored procedure can be modified to accept an XML fragment that contains one or more skill elements, thereby allowing the user to delete multiple skill nodes with a single invocation of the stored procedure.

/* Stored procedure to delete a specified skill element for a candidate */
CREATE PROCEDURE [DeleteSkillInfo]
   @JobCandidateID [int],
   @Skill [varchar](200)
AS
BEGIN
   SET NOCOUNT ON;

   UPDATE [CandidateInfoXMLDataType]
   SET Resume.modify('
   delete (/JobCandidate/Skills/Skill[.=sql:variable("@Skill")])
   ')
   WHERE JobCandidateID = @JobCandidateID
END

This stored procedure can easily be modified to accept an XML fragment which contains one or more skill elements thereby allowing the user to delete multiple skill nodes with a single invocation of stored procedure.

The Update Operation

The replace value of keyword enables you to modify the value of an existing node. To modify the value of an existing node, you have to specify an XQuery expression that identifies the node whose value is to be updated and another expression that specifies the new value of the node.

While modifying an untyped XML instance, the target expression should return a simply typed node. In the case of a typed XML instance, the target expression should evaluate to the same type or a subtype of the source expression.

Syntax:

replace value of 
   Expression1 
with
   Expression2

Example: Updating a skill

The following example shows how to update an existing skill value for a specified candidate using the replace value of keyword.

/* Stored procedure to update a specified skill element for a candidate */
CREATE PROCEDURE [UpdateSkillInfo]
   @JobCandidateID [int],
   @SkillOld [varchar](200),
   @SkillNew [varchar](200)
AS
BEGIN
   SET NOCOUNT ON;

   UPDATE [CandidateInfoXMLDataType]
   SET Resume.modify('
   replace value of  (/JobCandidate/Skills/Skill[.=sql:variable("@SkillOld")]/text())[1]
   with sql:variable("@SkillNew")
   ')
   WHERE JobCandidateID = @JobCandidateID
END

Unlike the delete operation, update and insert operations can only affect one node in one operation.

XQuery Usage Scenarios

Scenario 1: Performance Appraisal System

The Human Resources department of an organization needs a performance appraisal system which can handle typical appraisal-related activities for the company. Typically, appraisal records contain information that is mostly descriptive in nature such as employee performance measured against the key objective set for the appraisal period, defining key objectives for the next appraisal period, identifying employee training needs, and so on. XML is best suited for handling such information. The appraisal information for an employee can be stored in a column of type XML which would allow the users of the system to query and analyze the performance history of employees using XQuery. Furthermore, XML DML can be used to modify the appraisal records.

Let us assume that the training department of the organization is conducting a training session on ASP.NET and would like to invite all employees who requested training in ASP.NET. The following query selects all employees who opted for a training session on ASP.NET during their appraisal process.

SELECT Appraisal.query('
   for $PA in /PerformanceAppraisal,
       $Skill in $PA/TrainingNeeds/Technical/Skill 
   where contains($Skill, "ASP.NET")
   return
   element Employee
   {
     element EmpID { data($PA/Employee/EmployeeID) },
     element EmpName { data($PA/Employee/EmployeeName) },
     element EMail { data($PA/Employee/EMailID) },
     element Skill { data($Skill) }
   }
') as Result
FROM [EmployeePerformanceAppraisal]
WHERE Appraisal.exist('/PerformanceAppraisal/TrainingNeeds/Technical/Skill/text()[contains(.,"ASP.NET")]') = 1

Scenario 2: Medical Records System

A hospital needs a system which can capture medical information related to patients. This information includes patient details, insurance information, admission details, diagnosis information, treatment information, and so on. This information will be used for research or reference purposes. XML is the format of choice for storing this information as the patient medical data such as symptoms, lab reports, and treatment information contains descriptive information. Patient data can be stored in a column of type XML. The hospital can use XQuery for analyzing this information.

The following table and the XML instance are used to demonstrate the use of XQuery in a Patient Medical Records scenario:

Table:

CREATE TABLE [MedicalRecords](
   [PatientID] [int] IDENTITY(1,1) NOT NULL,
   [PatientRecord] [xml] NOT NULL,
   [ModifiedDate] [datetime] NOT NULL DEFAULT (getdate())
,
PRIMARY KEY CLUSTERED 
(
   [PatientID] ASC
) ON [PRIMARY]
) ON [PRIMARY]
GO
CREATE PRIMARY XML INDEX idx_PatientRecord on [MedicalRecords] (PatientRecord)
GO
CREATE XML INDEX idx_PatientRecord_Path on [MedicalRecords] (PatientRecord) USING XML INDEX idx_PatientRecord FOR PATH

Sample XML Instance:

<PatientRecord>
  <PatientDetails>
    <Name>Robert</Name>
    <Gender>Male</Gender>
    <Age>5</Age>
    <InsuranceInfo>
      <Company>Blue Cross Blue Shield</Company>
      <ID>D8456798</ID>
   </InsuranceInfo>
  </PatientDetails>
  <HospitalDetails>
    <Name>KK Hospital</Name>
    <Department>Pediatrics</Department>
  </HospitalDetails>
  <AdmissionDetails>
    <RegistrationNo>D4321</RegistrationNo>
    <DateAdmitted>2004-05-02</DateAdmitted>
    <DateDischarged>2004-05-08</DateDischarged>
  </AdmissionDetails>
  <ProblemDetails>
   <Symptoms>
      <Symptom>Abdominal pain</Symptom>
      <Symptom>Dehydration</Symptom>
   </Symptoms>
    <Diagnosis>Diarrhea</Diagnosis>
   <TreatmentInfo>
     <Therapy>Oral Rehydration Therapy</Therapy>
     <PrescriptionDetails>
       <Item>Pepto-Bismol</Item>
       <Item>Electrolyte Solutions</Item>
      </PrescriptionDetails>
   </TreatmentInfo>
  </ProblemDetails>
</PatientRecord>

Let us assume that a doctor is interested in viewing the medical records of patients who were admitted with the symptoms of "Fever" and "Abdominal Pain". The following query retrieves records of patients who were admitted with the symptoms of "Fever" and "Abdominal Pain".

SELECT PatientID, PatientRecord.query('
   element PatientName { data(/PatientRecord/PatientDetails/Name) },
   element MedicalInfo { /PatientRecord/ProblemDetails }
') as Result
FROM [MedicalRecords]
WHERE PatientRecord.exist('/PatientRecord/ProblemDetails/Symptoms/Symptom/text()[contains(.,"Fever")]') = 1
AND PatientRecord.exist('/PatientRecord/ProblemDetails/Symptoms/Symptom/text()[contains(.,"Abdominal Pain")]') = 1

Scenario 3: Asset Management System

The Information Technology department of an organization needs to develop an application which can manage assets such as hardware and software. This application should be capable of tracking information related to hardware assets including Asset ID, user details, system information such as processor, memory, BIOS, motherboard, video card details, software installed on the system, purchase information, and warranty information. The application should also maintain information related to the software assets of the organization such as software type, number of licenses purchased, etc. The application should be extensible to handle any new asset types that will be defined in the future. This asset management system will be used for generating reports such as hardware usage information, software license usage information, and hardware items due for maintenance. In the current scenario, there is a need for storing information confirming to different schemas in the same column of a table. Untyped XML can be used to store information with varying schemas. Typed XML with a schema collection can also be used to store such information. The asset information stored as an XML data type can be queried using XQuery.

Let us assume that the Information Technology department is interested in selecting a list of computer systems where both Microsoft Windows XP and Microsoft Office 2000 are installed. The following query selects records of hardware assets where both Windows XP and Office 2000 are installed.

SELECT AssetID, AssetDetails.query('
   <Asset>
   {
     element UserName { data(/AssetInfo/UserInfo/Name) },
     element ComputerName { data(/AssetInfo/SystemInfo/ComputerName) },
     element OS { data(/AssetInfo/SystemInfo/OS) }
   }
   </Asset>
') as Result
FROM [Assets]
WHERE Category = 'Hardware'
AND AssetDetails.exist('/AssetInfo/SystemInfo/OS/text()[contains(.,"Windows XP")]') = 1
AND AssetDetails.exist('/AssetInfo/SoftwaresInstalled/Software/text()[contains(.,"Office 2000")]') = 1

Conclusion

This paper serves as the starting point for readers who are interested in learning the basics of XQuery in the context of SQL Server 2005. Different features of the XQuery language that are implemented in SQL Server 2005 along with non-supported features are discussed in the paper. Examples illustrating the use of different features of XQuery language are also presented. The XQuery usage scenarios in this paper will help the user to identify scenarios where using XQuery is appropriate.

For more information:

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