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Guidelines for Using xml Data Type Methods

This topic describes guidelines for using the xml data type methods.

The xml data type methods cannot be used in the PRINT statement as shown in the following example. The xml data type methods are treated as subqueries, and subqueries are not allowed in the PRINT statement. As a result, the following example returns an error:

DECLARE @x xml
SET @x = '<root>Hello</root>'
PRINT @x.value('/root[1]', 'varchar(20)') -- will not work because this is treated as a subquery (select top 1 col from table) 

A solution is to first assign the result of the value() method to a variable of xml type and then specify the variable in the query.

DECLARE @x xml
DECLARE @c varchar(max)
SET @x = '<root>Hello</root>'
SET @c = @x.value('/root[1]', 'varchar(11)')
PRINT @c                                                      

The xml data type methods are treated internally as subqueries. Because GROUP BY requires a scalar and does not allow aggregates and subqueries, you cannot specify the xml data type methods in the GROUP BY clause. A solution is to call a user-defined function that uses XML methods inside of it.

When reporting errors, xml data type methods raise a single error in the following format:

Msg errorNumber, Level levelNumber, State stateNumber:
XQuery [database.table.method]: description_of_error

For example:

Msg 2396, Level 16, State 1:
XQuery [xmldb_test.xmlcol.query()]: Attribute may not appear outside of an element

Location steps, function parameters, and operators that require singletons will return an error if the compiler cannot determine whether a singleton is guaranteed at run time. This problem occurs frequently with untyped data. For example, the lookup of an attribute requires a singleton parent element. An ordinal that selects a single parent node is sufficient. The evaluation of a node()-value() combination to extract attribute values may not require the ordinal specification. This is shown in the next example.

Example: Known Singleton

In this example, the nodes() method generates a separate row for each <book> element. The value() method that is evaluated on a <book> node extracts the value of @genre and, being an attribute, is a singleton.

SELECT nref.value('@genre', 'varchar(max)') LastName
FROM   T CROSS APPLY xCol.nodes('//book') AS R(nref)

XML schema is used for type checking of typed XML. If a node is specified as a singleton in the XML schema, the compiler uses that information and no error occurs. Otherwise, an ordinal that selects a single node is required. In particular, the use of descendant-or-self axis (//) axis, such as in /book//title, looses singleton cardinality inference for the <title> element, even if the XML schema specifies it to be so. Therefore, you should rewrite it as (/book//title)[1].

It is important to remain aware of the difference between //first-name[1] and (//first-name)[1] for type checking. The former returns a sequence of <first-name> nodes in which each node is the leftmost <first-name> node among its siblings. The latter returns the first singleton <first-name> node in document order in the XML instance.

Example: Using value()

The following query on an untyped XML column results in a static, compilation error.This is because value() expects a singleton node as the first argument and the compiler cannot determine whether only one <last-name> node will occur at run time:

SELECT xCol.value('//author/last-name', 'nvarchar(50)') LastName
FROM   T

Following is a solution that you could consider:

SELECT xCol.value('//author/last-name[1]', 'nvarchar(50)') LastName
FROM   T

However, this solution does not solve the error, because multiple <author> nodes may occur in each XML instance. The following rewrite works:

SELECT xCol.value('(//author/last-name/text())[1]', 'nvarchar(50)') LastName
FROM   T

This query returns the value of the first <last-name> element in each XML instance.

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