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Conditional Expressions (XQuery)

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
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