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value() Method (xml Data Type)

  • Artikkeli

Performs an XQuery against the XML and returns a value of SQL type. This method returns a scalar value.

You typically use this method to extract a value from an XML instance stored in an xml type column, parameter, or variable. In this way, you can specify SELECT queries that combine or compare XML data with data in non-XML columns.

Syntax

value (XQuery, SQLType)

Arguments

  • XQuery
    Is the XQuery expression, a string literal, that retrieves data inside the XML instance. The XQuery must return at most one value. Otherwise, an error is returned.
  • SQLType
    Is the preferred SQL type, a string literal, to be returned. The return type of this method matches the SQLType parameter. SQLType cannot be an xml data type, a common language runtime (CLR) user-defined type, image, text, ntext, or sql_variant data type. SQLType can be an SQL, user-defined data type.

The value() method uses the Transact-SQL CONVERT operator implicitly and tries to convert the result of the XQuery expression, the serialized string representation, from XSD type to the corresponding SQL type specified by Transact-SQL conversion. For more information about type casting rules for CONVERT, see CAST and CONVERT (Transact-SQL).

Note

For performance reasons, instead of using the value() method in a predicate to compare with a relational value, use exist() with sql:column(). This is shown in example D that follows.

Examples

A. Using the value() method against an xml type variable

In the following example, an XML instance is stored in a variable of xml type. The value() method retrieves the ProductID attribute value from the XML. The value is then assigned to an int variable.

DECLARE @myDoc xml
DECLARE @ProdID int
SET @myDoc = '<Root>
<ProductDescription ProductID="1" ProductName="Road Bike">
<Features>
  <Warranty>1 year parts and labor</Warranty>
  <Maintenance>3 year parts and labor extended maintenance is available</Maintenance>
</Features>
</ProductDescription>
</Root>'

SET @ProdID =  @myDoc.value('(/Root/ProductDescription/@ProductID)[1]', 'int' )
SELECT @ProdID

Value 1 is returned as a result.

Although there is only one ProductID attribute in the XML instance, the static typing rules require you to explicitly specify that the path expression returns a singleton. Therefore, the additional [1] is specified at the end of the path expression. For more information about static typing, see XQuery and Static Typing.

B. Using the value() method to retrieve a value from an xml type column

The following query is specified against an xml type column (CatalogDescription) in the AdventureWorks database. The query retrieves ProductModelID attribute values from each XML instance stored in the column.

SELECT CatalogDescription.value('           
    declare namespace PD="https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelDescription";           
       (/PD:ProductDescription/@ProductModelID)[1]', 'int') AS Result           
FROM Production.ProductModel           
WHERE CatalogDescription IS NOT NULL           
ORDER BY Result desc

Note the following from the previous query:

  • The namespace keyword is used to define a namespace prefix.
  • Per static typing requirements, [1] is added at the end of the path expression in the value() method to explicitly indicate that the path expression returns a singleton.

This is the partial result:

-----------
35         
34         
...

C. Using the value() and exist() methods to retrieve values from an xml type column

The following example shows using both the value() method and the exist() method of the xml data type. The value() method is used to retrieve ProductModelID attribute values from the XML. The exist() method in the WHERE clause is used to filter the rows from the table.

The query retrieves product model IDs from XML instances that include warranty information (the <Warranty> element) as one of the features. The condition in the WHERE clause uses the exist() method to retrieve only the rows satisfying this condition.

SELECT CatalogDescription.value('
     declare namespace PD="https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelDescription";
           (/PD:ProductDescription/@ProductModelID)[1] ', 'int') as Result
FROM  Production.ProductModel
WHERE CatalogDescription.exist('
     declare namespace PD="https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelDescription";
     declare namespace wm="https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelWarrAndMain";

     /PD:ProductDescription/PD:Features/wm:Warranty ') = 1

Note the following from the previous query:

  • The CatalogDescription column is a typed XML column. This means that it has a schema collection associated with it. In the XQuery Prolog, the namespace declaration is used to define the prefix that is used later in the query body.
  • If the exist() method returns a 1 (True), it indicates that the XML instance includes the <Warranty> child element as one of the features.
  • The value() method in the SELECT clause then retrieves the ProductModelID attribute values as integers.

This is the partial result:

Result     
-----------
19         
23         
...

D. Using the exist() method instead of the value() method

For performance reasons, instead of using the value() method in a predicate to compare with a relational value, use exist() with sql:column(). For example:

CREATE TABLE T (c1 int, c2 varchar(10), c3 xml)
GO

SELECT c1, c2, c3 
FROM T
WHERE c3.value( '/root[1]/@a', 'integer') = c1
GO

This can be written in the following way:

SELECT c1, c2, c3 
FROM T
WHERE c3.exist( '/root[@a=sql:column("c1")]') = 1
GO

See Also

Concepts

Adding Namespaces Using WITH XMLNAMESPACES
Typed vs. Untyped XML
xml Data Type
Generating XML Instances
XML Data Modification Language (XML DML)
Sample XML Applications

Other Resources

xml Data Type Methods

Help and Information

Getting SQL Server 2005 Assistance