Conditional Expressions (XQuery)

Conditional Expressions (XQuery)

 

Applies To: SQL Server

XQuery supports the following conditional if-then-else statement:

if (<expression1>)  
then  
  <expression2>  
else  
  <expression3>  

Depending on the effective Boolean value of expression1, either expression2 or expression3 is evaluated. For example:

  • If the test expression, expression1, results in an empty sequence, the result is False.

  • If the test expression, expression1, results in a simple Boolean value, this value is the result of the expression.

  • If the test expression, expression1, results in a sequence of one or more nodes, the result of the expression is True.

  • Otherwise, a static error is raised.

Also note the following:

  • The test expression must be enclosed between parentheses.

  • The else expression is required. If you do not need it, you can return " ( ) ", as illustrated in the examples in this topic.

For example, the following query is specified against the xml type variable. The if condition tests the value of the SQL variable (@v) inside the XQuery expression by using the sql:variable() function extension function. If the variable value is "FirstName", it returns the <FirstName> element. Otherwise, it returns the <LastName> element.

declare @x xml  
declare @v varchar(20)  
set @v='FirstName'  
set @x='  
<ROOT rootID="2">  
  <FirstName>fname</FirstName>  
  <LastName>lname</LastName>  
</ROOT>'  
SELECT @x.query('  
if ( sql:variable("@v")="FirstName" ) then  
  /ROOT/FirstName  
 else  
   /ROOT/LastName  
')  

This is the result:

<FirstName>fname</FirstName>  

The following query retrieves the first two feature descriptions from the product catalog description of a specific product model. If there are more features in the document, it adds a <there-is-more> element with empty content.

SELECT CatalogDescription.query('  
     declare namespace p1="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelDescription";  
     <Product>   
          { /p1:ProductDescription/@ProductModelID }  
          { /p1:ProductDescription/@ProductModelName }   
          {  
            for $f in /p1:ProductDescription/p1:Features/*[position()<=2]  
            return  
            $f   
          }  
          {  
            if (count(/p1:ProductDescription/p1:Features/*) > 2)  
            then <there-is-more/>  
            else ()  
          }   
     </Product>          
') as x  
FROM Production.ProductModel  
WHERE ProductModelID = 19  

In the previous query, the condition in the if expression checks whether there are more than two child elements in <Features>. If yes, it returns the <there-is-more/> element in the result.

This is the result:

<Product ProductModelID="19" ProductModelName="Mountain 100">  
  <p1:Warranty xmlns:p1="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelWarrAndMain">  
    <p1:WarrantyPeriod>3 years</p1:WarrantyPeriod>  
    <p1:Description>parts and labor</p1:Description>  
  </p1:Warranty>  
  <p2:Maintenance xmlns:p2="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelWarrAndMain">  
    <p2:NoOfYears>10 years</p2:NoOfYears>  
    <p2:Description>maintenance contract available through your dealer or any AdventureWorks retail store.</p2:Description>  
  </p2:Maintenance>  
  <there-is-more />  
</Product>  

In the following query, a <Location> element with a LocationID attribute is returned if the work center location does not specify the setup hours.

SELECT Instructions.query('  
     declare namespace AWMI="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelManuInstructions";  
        for $WC in //AWMI:root/AWMI:Location  
        return  
        if ( $WC[not(@SetupHours)] )  
        then  
          <WorkCenterLocation>  
             { $WC/@LocationID }   
          </WorkCenterLocation>  
         else  
           ()  
') as Result  
FROM Production.ProductModel  
where ProductModelID=7  

This is the result:

<WorkCenterLocation LocationID="30" />  
<WorkCenterLocation LocationID="45" />  
<WorkCenterLocation LocationID="60" />  

This query can be written without the if clause, as shown in the following example:

SELECT Instructions.query('  
     declare namespace AWMI="http://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelManuInstructions";  
for $WC in //AWMI:root/AWMI:Location[not(@SetupHours)]   
        return  
          <Location>  
             { $WC/@LocationID }   
          </Location>  
') as Result  
FROM Production.ProductModel  
where ProductModelID=7  

XQuery Expressions

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