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XML Schema Collections (SQL Server)

As described in the topic, xml (Transact-SQL), SQL Server provides native storage of XML data through the xml data type. You can optionally associate XSD schemas with a variable or a column of xml type through an XML schema collection. The XML schema collection stores the imported XML schemas and is then used to do the following:

  • Validate XML instances

  • Type the XML data as it is stored in the database

Note that the XML schema collection is a metadata entity like a table in the database. You can create, modify, and drop them. Schemas specified in a CREATE XML SCHEMA COLLECTION (Transact-SQL) statement are automatically imported into the newly created XML schema collection object. You can import additional schemas or schema components into an existing collection object in the database by using the ALTER XML SCHEMA COLLECTION (Transact-SQL) statement.

As described in the topic, Typed vs. Untyped XML, the XML stored in a column or variable that a schema is associated with is referred to as typed XML, because the schema provides the necessary data type information for the instance data. SQL Server uses this type information to optimize data storage.

The query-processing engine also uses the schema for type checking and to optimize queries and data modification.

Also, SQL Server uses the associated XML schema collection, in the case of typed xml, to validate the XML instance. If the XML instance complies with the schema, the database allows the instance to be stored in the system with their type information. Otherwise, it rejects the instance.

You can use the intrinsic function XML_SCHEMA_NAMESPACE to retrieve the schema collection that is stored in the database. For more information, see View a Stored XML Schema Collection.

You can also use the XML schema collection to type XML variables, parameters, and columns.

You can create XML schema collections in the database and associate them with variables and columns of xml type. To manage schema collections in the database, SQL Server provides the following DDL statements:

To use an XML schema collection and the schemas it contains, you must first create the collection and the schemas by using the CREATE XML SCHEMA COLLECTION statement. After the schema collection is created, you can then create variables and columns of xml type and associate the schema collection with them. Note that after a schema collection is created, various schema components are stored in the metadata. You can also use the ALTER XML SCHEMA COLLECTION to add more components to the existing schemas or add new schemas to an existing collection.

To drop the schema collection, use the DROP XML SCHEMA COLLECTION statement. This drops all schemas that are contained in the collection and removes the collection object. Note that before you can drop a schema collection, the conditions described in DROP XML SCHEMA COLLECTION (Transact-SQL)must be met.

When you use the CREATE XML SCHEMA COLLECTION statement, various schema components are imported into the database. Schema components include schema elements, attributes, and type definitions. When you use the DROP XML SCHEMA COLLECTION statement, you remove the complete collection.

CREATE XML SCHEMA COLLECTION saves the schema components into various system tables.

For example, consider the following schema:

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
            targetNamespace="uri:Cust_Orders2"
            xmlns="uri:Cust_Orders2" >
  <xsd:attribute name="SomeAttribute" type="xsd:int" />
  <xsd:complexType name="SomeType" />
  <xsd:complexType name="OrderType" >
    <xsd:sequence>
      <xsd:element name="OrderDate" type="xsd:date" />
      <xsd:element name="RequiredDate" type="xsd:date" />
      <xsd:element name="ShippedDate" type="xsd:date" />
    </xsd:sequence>
    <xsd:attribute name="OrderID" type="xsd:ID" />
    <xsd:attribute name="CustomerID"  />
    <xsd:attribute name="EmployeeID"  />
  </xsd:complexType>
  <xsd:complexType name="CustomerType" >
     <xsd:sequence>
        <xsd:element name="Order" type="OrderType"
                     maxOccurs="unbounded" />
       </xsd:sequence>
      <xsd:attribute name="CustomerID" type="xsd:string" />
      <xsd:attribute name="OrderIDList" type="xsd:IDREFS" />
  </xsd:complexType>
  <xsd:element name="Customer" type="CustomerType" />
</xsd:schema>

The previous schema shows the different types of components that can be stored in the database. These include SomeAttribute, SomeType, OrderType, CustomerType, Customer, Order, CustomerID, OrderID, OrderDate, RequiredDate, and ShippedDate.

Component Categories

The Schema components stored in the database fall into the following categories:

  • ELEMENT

  • ATTRIBUTE

  • TYPE (for simple or complex types)

  • ATTRIBUTEGROUP

  • MODELGROUP

For example:

  • SomeAttribute is an ATTRIBUTE component.

  • SomeType, OrderType, and CustomerType are TYPE components.

  • Customer is an ELEMENT component.

When you import a schema into the database, SQL Server does not store the schema itself. Instead, SQL Server stores the various individual components. That is, the <Schema> tag is not stored, only the components that are defined within it are preserved. All schema elements are not preserved. If the <Schema> tag contains attributes that specify default behavior of its components, these attributes are moved to the schema components within it during the import process, as shown in the following table.

Attribute name

Behavior

attributeFormDefault

The form attribute applied to all attribute declarations in the schema where it is not already present and the value is set to the value of the attributeFormDefault attribute.

elementFormDefault

The form attribute applied to all element declarations in the schema where it is not already present and the value is set to the value of the elementFormDefault attribute.

blockDefault

The block attribute applied to all element declarations and type definitions where it is not already present and the value is set to the value of the blockDefault attribute.

finalDefault

The final attribute applied to all element declarations and type definitions where it is not already present and the value is set to the value of the finalDefault attribute.

targetNamespace

Information about the components that belong to the target namespace is stored in the metadata.

You must have the necessary permissions to do the following:

  • Create/load the XML schema collection

  • Modify the XML schema collection

  • Drop the XML schema collection

  • Use the XML schema collection to type xml type columns, variables, and parameters, or use it in table or column constraints

The SQL Server security model allows CONTROL permission on every object. The grantee of this permission obtains all other permissions on the object. The owner of the object also has all the permissions on the object.

The owner and the grantee of the CONTROL permission on an object can grant any permission on the object. A user who is not the owner and does not have CONTROL permission can still grant permission on an object when WITH GRANT OPTION is specified. For example, assume User A has REFERENCES permission on XML schema collection S, through WITH GRANT OPTION, but no other permissions on S. User A could grant User B REFERENCES permission on schema collection S.

The security model also allows permissions to create and use XML schema collections or transfer ownership from one user to another. The following topics describe the XML schema collection permissions.

XML schema collections are enumerated in the catalog view, sys.xml_schema_collections. The XML schema collection "sys" is defined by the system. It contains the predefined namespaces that can be used in all user-defined XML schema collections without having to load them explicitly. This list contains the namespaces for xml, xs, xsi, fn, and xdt. Two other catalog views are sys.xml_schema_namespaces, which enumerates all namespaces within each XML schema collection, and sys.xml_components, which enumerates all XML schema components within each XML schema.

The built-in function XML_SCHEMA_NAMESPACE, schemaName, XmlSchemacollectionName, namespace-uri, yields an xml data type instance.. This instance contains XML schema fragments for schemas that are contained in an XML schema collection, except the predefined XML schemas.

You can enumerate the contents of an XML schema collection in the following ways:

  • Write Transact-SQL queries on the appropriate catalog views for XML schema collections.

  • Use the built-in function XML_SCHEMA_NAMESPACE(). You can apply xml data type methods on the output of this function. However, you cannot modify the underlying XML schemas.

These are illustrated in the following examples.

Example: Enumerate the XML Namespaces in an XML Schema Collection

Use the following query for the XML schema collection "myCollection":

SELECT XSN.name
FROM    sys.xml_schema_collections XSC JOIN sys.xml_schema_namespaces XSN
    ON (XSC.xml_collection_id = XSN.xml_collection_id)
WHERE    XSC.name = 'myCollection'   

Example: Enumerate the Contents of an XML Schema Collection

The following statement enumerates the contents of the XML schema collection "myCollection" within the relational schema, dbo.

SELECT XML_SCHEMA_NAMESPACE (N'dbo', N'myCollection')

Individual XML schemas within the collection can be obtained as xml data type instances by specifying the target namespace as the third argument to XML_SCHEMA_NAMESPACE(). This is shown in the following example.

Example: Output a Specified Schema from an XML Schema Collection

The following statement outputs the XML schema with the target namespace "http://www.microsoft.com/books" from the XML schema collection "myCollection" within the relational schema, dbo.

SELECT XML_SCHEMA_NAMESPACE (N'dbo', N'myCollection', 
N'http://www.microsoft.com/books')

Querying XML Schemas

You can query XML schemas that you have loaded into XML schema collections in the following ways:

  • Write Transact-SQL queries on catalog views for XML schema namespaces.

  • Create a table that contains an xml data type column to store your XML schemas and also load them into the XML type system. You can query the XML column by using the xml data type methods. Also, you can build an XML index on this column. However, with this approach, the application must maintain consistency between the XML schemas stored in the XML column and the XML type system. For example, if you drop the XML schema namespace from the XML type system, you also have to drop it from the table in order to preserve consistency.

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