exist() Method (xml Data Type)

Artikel
09/03/2024

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

Returns a bit that represents one of the following conditions:

1, representing True, if the XQuery expression in a query returns a nonempty result. That is, it returns at least one XML node.
0, representing False, if it returns an empty result.
NULL if the xml data type instance against which the query was executed contains NULL.

Syntax

exist (XQuery)

Arguments

XQuery
Is an XQuery expression, a string literal.

Remarks

Note

The exist() method returns 1 for the XQuery expression that returns a nonempty result. If you specify the true() or false() functions inside the exist() method, the exist() method will return 1, because the functions true() and false() return Boolean True and False, respectively. That is, they return a nonempty result). Therefore, exist() will return 1 (True), as shown in the following example:

DECLARE @x XML;  
SET @x='';  
SELECT @x.exist('true()');

Examples

The following examples show how to specify the exist() method.

Example: Specifying the exist() method against an xml type variable

In the following example, @x is an xml type variable (untyped xml) and @f is an integer type variable that stores the value returned by the exist() method. The exist() method returns True (1) if the date value stored in the XML instance is 2002-01-01.

DECLARE @x XML;  
DECLARE @f BIT;  
SET @x = '<root Somedate = "2002-01-01Z"/>';  
SET @f = @x.exist('/root[(@Somedate cast as xs:date?) eq xs:date("2002-01-01Z")]');  
SELECT @f;

In comparing dates in the exist() method, note the following:

The code cast as xs:date? is used to cast the value to xs:date type for purposes of comparison.
The value of the @Somedate attribute is untyped. In comparing this value, it is implicitly cast to the type on the right side of the comparison, the xs:date type.
Instead of cast as xs:date(), you can use the xs:date() constructor function. For more information, see Constructor Functions (XQuery).

The following example is similar to the previous one, except it has a <Somedate> element.

DECLARE @x XML;  
DECLARE @f BIT;  
SET @x = '<Somedate>2002-01-01Z</Somedate>';  
SET @f = @x.exist('/Somedate[(text()[1] cast as xs:date ?) = xs:date("2002-01-01Z") ]')  
SELECT @f;

Note the following from the previous query:

The text() method returns a text node that contains the untyped value 2002-01-01. (The XQuery type is xdt:untypedAtomic.) You must explicitly cast this typed value from x to xsd:date, because implicit casting is not supported in this case.

Example: Specifying the exist() method against a typed xml variable

The following example illustrates the use of the exist() method against an xml type variable. It is a typed XML variable, because it specifies the schema namespace collection name, ManuInstructionsSchemaCollection.

In the example, a manufacturing instructions document is first assigned to this variable and then the exist() method is used to find whether the document includes a <Location> element whose LocationID attribute value is 50.

The exist() method specified against the @x variable returns 1 (True) if the manufacturing instructions document includes a <Location> element that has LocationID=50. Otherwise, the method returns 0 (False).

DECLARE @x XML (Production.ManuInstructionsSchemaCollection);  
SELECT @x=Instructions  
FROM Production.ProductModel  
WHERE ProductModelID=67;  
--SELECT @x  
DECLARE @f INT;  
SET @f = @x.exist(' declare namespace AWMI="https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelManuInstructions";  
    /AWMI:root/AWMI:Location[@LocationID=50]  
');  
SELECT @f;

Example: Specifying the exist() method against an xml type column

The following query retrieves product model IDs whose catalog descriptions do not include the specifications, <Specifications> element:

SELECT ProductModelID, CatalogDescription.query('  
declare namespace pd="https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelDescription";  
    <Product   
        ProductModelID= "{ sql:column("ProductModelID") }"   
        />  
') AS Result  
FROM Production.ProductModel  
WHERE CatalogDescription.exist('  
    declare namespace  pd="https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelDescription";  
     /pd:ProductDescription[not(pd:Specifications)]'  
    ) = 1;

Note the following from the previous query:

The WHERE clause selects only those rows from the ProductDescription table that satisfy the condition specified against the CatalogDescription xml type column.
The exist() method in the WHERE clause returns 1 (True) if the XML does not include any <Specifications> element. Note the use of the not() function (XQuery).
The sql:column() function (XQuery) function is used to bring in the value from a non-XML column.
This query returns an empty rowset.

The query specifies query() and exist() methods of the xml data type and both these methods declare the same namespaces in the query prolog. In this case, you may want to use WITH XMLNAMESPACES to declare the prefix and use it in the query.

WITH XMLNAMESPACES ('https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelDescription' AS pd)  
SELECT ProductModelID, CatalogDescription.query('  
    <Product   
        ProductModelID= "{ sql:column("ProductModelID") }"   
        />  
') AS Result  
FROM Production.ProductModel  
WHERE CatalogDescription.exist('  
     /pd:ProductDescription[not(pd:Specifications)]'  
    ) = 1;

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