Expression Context and Query Evaluation (XQuery)

Static Context

Static context initialization refers to the process of putting together all the information for static analysis of the expression. As part of static context initialization, the following is completed:

• The boundary white space policy is set to strip. Therefore, the boundary white space is not preserved by the any element and attribute constructors in the query. For example:

  declare @x xml
  set @x=''
  select @x.query('<a>  {"Hello"}  </a>,
  
      <b> {"Hello2"}  </b>')
  

  This query returns the following result, because the boundary space is stripped away during parsing of the XQuery expression:

  <a>Hello</a><b>Hello2</b>
  

• The prefix and the namespace binding are initialized for the following:

  • A set of predefined namespaces.

  • Any namespaces defined using WITH XMLNAMESPACES. For more information, see Adding Namespaces Using WITH XMLNAMESPACES).

  • Any namespaces defined in the query prolog. Note that the namespace declarations in the prolog may override the namespace declaration in the WITH XMLNAMESPACES. For example, in the following query, WITH XMLNAMESPACES declares a prefix (pd) that binds it to namespace (http://someURI). However, in the WHERE clause, the query prolog overrides the binding.

    WITH XMLNAMESPACES ('http://someURI' AS pd)
    SELECT ProductModelID, CatalogDescription.query('
        <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[(pd:Specifications)]'
        ) = 1
    

  All these namespace bindings are resolved during static context initialization.

• If querying a typed xml column or variable, the components of the XML schema collection associated with the column or variable are imported into the static context. For more information, see Typed XML Compared to Untyped XML.

• For every atomic type in the imported schemas, a casting function is also made available in the static context. This is illustrated in the following example. In this example, a query is specified against a typed xml variable. The XML schema collection associated with this variable defines an atomic type, myType. Corresponding to this type, a casting function, myType(), is available during the static analysis. The query expression (ns:myType(0)) returns a value of myType.

  -- DROP XML SCHEMA COLLECTION SC
  -- go
  CREATE XML SCHEMA COLLECTION SC AS '<schema xmlns="http://www.w3.org/2001/XMLSchema" 
  targetNamespace="myNS" xmlns:ns="myNS"
  xmlns:s="https://schemas.microsoft.com/sqlserver/2004/sqltypes">
        <import namespace="https://schemas.microsoft.com/sqlserver/2004/sqltypes"/>
        <simpleType name="myType">
              <restriction base="int">
               <enumeration value="0" />
                <enumeration value="1"/>
              </restriction>
        </simpleType>
        <element name="root" type="ns:myType"/>
  </schema>'
  go
  
  DECLARE @var XML(SC)
  SET @var = '<root xmlns="myNS">0</root>'
  -- specify myType() casting function in the query
  SELECT @var.query('declare namespace ns="myNS"; ns:myType(0)')
  

  In the following example, the casting function for the int built-in XML type is specified in the expression.

  declare @x xml
  set @x = ''
  select @x.query('xs:int(5)')
  go
  

After the static context is initialized, the query expression is analyzed (compiled). The static analysis involves the following:

1. Query parsing.

2. Resolving the function and type names specified in the expression.

3. Static typing of the query. This makes sure that the query is type safe. For example, the following query returns a static error, because the + operator requires numeric primitive type arguments:

   declare @x xml
   set @x=''
   SELECT @x.query('"x" + 4')
   

   In the following example, the value() operator requires a singleton. As specified in the XML schema, there can be multiple <Elem> elements. Static analysis of the expression determines that it is not type safe and a static error is returned. To resolve the error, the expression must be rewritten to explicitly specify a singleton (data(/x:Elem)[1]).

   DROP XML SCHEMA COLLECTION SC
   go
   CREATE XML SCHEMA COLLECTION SC AS '<schema xmlns="http://www.w3.org/2001/XMLSchema" 
   targetNamespace="myNS" xmlns:ns="myNS"
   xmlns:s="https://schemas.microsoft.com/sqlserver/2004/sqltypes">
         <import namespace="https://schemas.microsoft.com/sqlserver/2004/sqltypes"/>
         <element name="Elem" type="string"/>
   </schema>'
   go
   
   declare @x xml (SC)
   set @x='<Elem xmlns="myNS">test</Elem><Elem xmlns="myNS">test2</Elem>'
   SELECT @x.value('declare namespace x="myNS"; data(/x:Elem)[1]','varchar(20)')
   

   For more information, see XQuery and Static Typing.

Implementation Restrictions

Dynamic Context

Dynamic context refers to the information that must be available at the time the expression is executed. In addition to the static context, the following information is initialized as part of dynamic context:

• The expression focus, such as the context item, context position, and context size, is initialized as shown in the following. Note that all these values can be overridden by the nodes() method.

  • The xml data type sets the context item, the node being processed, to the document node.

  • The context position, the position of the context item relative to the nodes being processed, is first set to 1.

  • The context size, the number of items in the sequence being processed, is first set to 1, because there is always one document node.

Implementation Restrictions

See Also

Reference

Concepts

Other Resources