---
title: "Reference the Built-in XML Schema Collection (sys) | Microsoft Docs"
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---
# Reference the Built-in XML Schema Collection (sys)
[!INCLUDE[appliesto-ss-asdb-xxxx-xxx-md](../../includes/appliesto-ss-asdb-xxxx-xxx-md.md)]
  Every database you create has a predefined **sys** XML schema collection in the **sys** relational schema. It reserves these predefined schemas, and they can be accessed from any other user-created XML schema collection. The prefixes used in these predefined schemas are meaningful in XQuery. Only **xml** is a reserved prefix.  
  
```  
xml = http://www.w3.org/XML/1998/namespace  
xs = http://www.w3.org/2001/XMLSchema  
xsi = http://www.w3.org/2001/XMLSchema-instance  
fn = http://www.w3.org/2004/07/xpath-functions  
sqltypes = https://schemas.microsoft.com/sqlserver/2004/sqltypes  
xdt = http://www.w3.org/2004/07/xpath-datatypes  
(no prefix) = urn:schemas-microsoft-com:xml-sql  
(no prefix) = https://schemas.microsoft.com/sqlserver/2004/SOAP  
```  
  
 Note that the **sqltypes** namespace contains components that can be referenced from any user-created XML schema collection. You can download the **sqltypes** schema from this [Microsoft Web site](https://go.microsoft.com/fwlink/?linkid=31850). The built-in components include the following:  
  
-   XSD types  
  
-   XML attributes **lang**, **base**, and **space**  
  
-   Components of the **sqltypes** namespace  
  
 The following query returns built-in components that can be referenced from a user-created XML schema collection:  
  
```  
SELECT C.name, N.name, C.symbol_space_desc from sys.xml_schema_components C join sys.xml_schema_namespaces N  
on ((C.xml_namespace_id = N.xml_namespace_id) AND (C.xml_collection_id = N.xml_collection_id))  
join sys.xml_schema_collections SC  
on SC.xml_collection_id = C.xml_collection_id  
where ((C.xml_collection_id = 1) AND (C.name is not null) AND (C.scoping_xml_component_id is null)   
AND (SC.schema_id = 4))  
GO  
```  
  
 The following example shows how these components are referenced in a user schema. `CREATE XML SCHEMA COLLECTION` creates an XML schema collection that references the `varchar` type defined in the `sqltypes` namespace. The example also references the `lang` attribute that is defined in the `xml` namespace.  
  
```  
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="http://www.w3.org/XML/1998/namespace"/>  
   <import namespace="https://schemas.microsoft.com/sqlserver/2004/sqltypes"/>  
   <element name="root">  
      <complexType>  
          <sequence>  
             <element name="a" type="string"/>  
             <element name="b" type="string"/>  
             <!-- varchar type is defined in the sys.sys collection and   
                  can be referenced in any user-defined schema -->  
             <element name="c" type="s:varchar"/>  
          </sequence>  
          <attribute name="att" type="int"/>  
          <!-- xml:lang attribute is defined in the sys.sys collection   
               and can be referenced in any user-defined schema -->  
          <attribute ref="xml:lang"/>  
      </complexType>  
    </element>  
 </schema>'  
GO  
 -- Cleanup  
DROP xml schema collection SC   
GO  
```  
  
 You should note the following:  
  
-   You cannot modify XML schemas with these namespaces in any user-defined XML schema collection. For example, the following XML schema collection fails, because it is adding a component to the `sqltypes` protected namespace:  
  
    ```  
    CREATE XML SCHEMA COLLECTION SC AS '  
    <schema xmlns="http://www.w3.org/2001/XMLSchema"   
    targetNamespace    
        ="https://schemas.microsoft.com/sqlserver/2004/sqltypes" >   
          <element name="root" type="string"/>  
    </schema>'  
    GO  
    ```  
  
-   You cannot use the `sys` XML schema collection to type `xml` columns, variables, or parameters. For example, the following code returns an error:  
  
    ```  
    DECLARE @x xml (sys.sys)  
    ```  
  
-   Serialization of these built-in schemas is not supported. For example, the following code returns an error:  
  
    ```  
    SELECT XML_SCHEMA_NAMESPACE(N'sys',N'sys')  
    GO  
    ```  
  
 The following code is another example in which you create an XML schema collection that uses the `varchar` type defined in the `sqltypes` namespace:  
  
```  
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="s:varchar">  
                  <maxLength value="20"/>  
            </restriction>  
      </simpleType>  
      <element name="root" type="ns:myType"/>  
</schema>'  
go  
```  
  
 As shown in the following, you can create a typed `XML` variable, assign an XML instance to it, and verify that the value of the <`root`> element type is a `varchar` type.  
  
```  
DECLARE @var XML(SC)  
SET @var = '<root xmlns="myNS">My data</root>'  
SELECT @var.query('declare namespace sqltypes = "https://schemas.microsoft.com/sqlserver/2004/sqltypes";  
declare namespace ns="myNS";   
data(/ns:root[1]) instance of sqltypes:varchar?')  
GO  
```  
  
 The `instance of sqltypes:varchar?` expression returns TRUE, because the <`root`> element value is of a type derived from **varchar** according to the schema that is associated with the `@var` variable.  
  
## See Also  
 [XML Schema Collections &#40;SQL Server&#41;](../../relational-databases/xml/xml-schema-collections-sql-server.md)