Type System (XQuery)
Applies to: 
SQL Server
XQuery is a strongly-typed language for schema types and a weakly-typed language for untyped data. The predefined types of XQuery include the following:
Built-in types of XML schema in the http://www.w3.org/2001/XMLSchema namespace.
Types defined in the http://www.w3.org/2004/07/xpath-datatypes namespace.
This topic also describes the following:
The typed value versus the string value of a node.
The data Function (XQuery) and the string Function (XQuery).
Matching the sequence type returned by an expression.
Built-in Types of XML Schema
The built-in types of XML schema have a predefined namespace prefix of xs. Some of these types include xs:integer and xs:string. All these built-in types are supported. You can use these types when you create an XML schema collection.
When querying typed XML, the static and dynamic type of the nodes is determined by the XML schema collection associated with the column or variable that is being queried. For more information about static and dynamic types, see Expression Context and Query Evaluation (XQuery). For example, the following query is specified against a typed xml column (Instructions). The expression uses instance of to verify that the typed value of the LotSize attribute returned is of xs:decimal type.
SELECT Instructions.query('
DECLARE namespace AWMI="https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelManuInstructions";
data(/AWMI:root[1]/AWMI:Location[@LocationID=10][1]/@LotSize)[1] instance of xs:decimal
') AS Result
FROM Production.ProductModel
WHERE ProductModelID=7
This typing information is provided by the XML schema collection associated with the column.
Types Defined in XPath Data Types Namespace
The types defined in the http://www.w3.org/2004/07/xpath-datatypes namespace have a predefined prefix of xdt. The following applies to these types:
You cannot use these types when you are creating an XML schema collection. These types are used in the XQuery type system and are used for XQuery and Static Typing. You can cast to the atomic types, for example, xdt:untypedAtomic, in the xdt namespace.
When querying untyped XML, the static and dynamic type of element nodes is xdt:untyped, and the type of attribute values is xdt:untypedAtomic. The result of a query() method generates untyped XML. This means that the XML nodes are returned as xdt:untyped and xdt:untypedAtomic, respectively.
The xdt:dayTimeDuration and xdt:yearMonthDuration types are not supported.
In the following example, the query is specified against an untyped XML variable. The expression, data(/a[1]), returns a sequence of one atomic value. The data() function returns the typed value of the element <a>. Because the XML being queried is untyped, the type of the value returned is xdt:untypedAtomic. Therefore, instance of returns true.
DECLARE @x xml
SET @x='<a>20</a>'
SELECT @x.query( 'data(/a[1]) instance of xdt:untypedAtomic' )
Instead of retrieving the typed value, the expression (/a[1]) in the following example returns a sequence of one element, element <a>. The instance of expression uses the element test to verify that the value returned by the expression is an element node of xdt:untyped type.
DECLARE @x xml
SET @x='<a>20</a>'
-- Is this an element node whose name is "a" and type is xdt:untyped.
SELECT @x.query( '/a[1] instance of element(a, xdt:untyped?)')
-- Is this an element node of type xdt:untyped.
SELECT @x.query( '/a[1] instance of element(*, xdt:untyped?)')
-- Is this an element node?
SELECT @x.query( '/a[1] instance of element()')
Note
When you are querying a typed XML instance and the query expression includes the parent axis, the static type information of the resulting nodes is no longer available. However, the dynamic type is still associated with the nodes.
Typed Value vs. String Value
Every node has a typed value and a string value. For typed XML data, the type of the typed value is provided by the XML schema collection associated with the column or variable that is being queried. For untyped XML data, the type of the typed value is xdt:untypedAtomic.
You can use the data() or string() function to retrieve the value of a node:
The data Function (XQuery) returns the typed value of a node.
The string Function (XQuery) returns the string value of the node.
In the following XML schema collection, the <root> element of the integer type is defined:
CREATE XML SCHEMA COLLECTION SC AS N'
<schema xmlns="http://www.w3.org/2001/XMLSchema">
<element name="root" type="integer"/>
</schema>'
GO
In the following example, the expression first retrieves the typed value of /root[1] and then adds 3 to it.
DECLARE @x xml(SC)
SET @x='<root>5</root>'
SELECT @x.query('data(/root[1]) + 3')
In the next example, the expression fails, because the string(/root[1]) in the expression returns a string type value. This value is then passed to an arithmetic operator that takes only numeric type values as its operands.
-- Fails because the argument is string type (must be numeric primitive type).
DECLARE @x xml(SC)
SET @x='<root>5</root>'
SELECT @x.query('string(/root[1]) + 3')
The following example computes the total of the LaborHours attributes. The data() function retrieves the typed values of LaborHours attributes from all the <Location> elements for a product model. According to the XML schema associated with the Instruction column, LaborHours is of xs:decimal type.
SELECT Instructions.query('
DECLARE namespace AWMI="https://schemas.microsoft.com/sqlserver/2004/07/adventure-works/ProductModelManuInstructions";
sum(data(//AWMI:Location/@LaborHours))
') AS Result
FROM Production.ProductModel
WHERE ProductModelID=7
This query returns 12.75 as the result.
Note
The explicit use of the data() function in this example is for illustration only. If it is not specified, sum() implicitly applies the data() function to extract the typed values of the nodes.