XML schemas provide a means to define the structure
and content of an XML document. SQL Server supports a subset of the XML
Schema and uses three DDL statements to manage XML schema collections: CREATE XML SCHEMA COLLECTION, ALTER XML SCHEMA COLLECTION, and DROP XML SCHEMA COLLECTION.
XML schema collections are XML schemas that are imported into a SQL
Server database and are defined using the XSD (XML Schema Definition)
language.
We will first explore the CREATE XML SCHEMA COLLECTION DDL statement. The CREATE XML SCHEMA COLLECTION DDL uses the syntax shown in Figure 1. Refer to Table 1 for arguments and descriptions.
Figure 1. CREATE XML SCHEMA COLLECTION Syntax
CREATE XML SCHEMA COLLECTION [ <relational_schema>. ]sql_identifier AS
Expression
|
Table 1. CREATE XML SCHEMA COLLECTION Arguments
| Arguments | Description |
|---|
| Relational_schema | Identifies the relational schema name. If not specified, default relational schema is assumed. |
| sql_identifier | Is the SQL identifier for the XML schema collection. |
| Expression | Is a string constant or scalar variable. Is varchar, varbinary, nvarchar, or XML type. |
Let’s examine the CREATE XML SCHEMA COLLECTION. In Figure 2, we created a sample XML schema collection using the CREATE XML SCHEMA COLLECTION DDL statement.
Figure 2. Sample CREATE XML SCHEMA COLLECTION
CREATE XML SCHEMA COLLECTION customerSchema AS
'<schema xmlns="http://www.w3.org/2001/XMLSchema">
<element name="root" type="string"/>
</schema>'
|
After the schema
collection is created, you can then assign it to an XML variable or a
column with the schema declarations it contains. You do this by
referencing the schema collection in the CREATE TABLE statement shown in Figure 3.
Figure 3. CREATE TABLE statement defining the XML SCHEMA COLLECTION
CREATE TABLE [dbo].[XML_Table](
[pk] [int] IDENTITY(1,1) NOT NULL,
[customerName] [varchar](255) NULL,
[customerPhone] [varchar](255) NULL,
[customerAddress] [xml](customerSchema) NULL,
CONSTRAINT [PK_XML_Table] PRIMARY KEY CLUSTERED
(
[pk] ASC
))
|
The ALTER XML
SCHEMA COLLECTION allows you to add new schema components to an existing
XML schema collection. The ALTER XML SCHEMA COLLECTION DDL uses the
following syntax as shown in Figure 4. Table 2 lists all the arguments and descriptions for the ALTER XML SCHEMA COLLECTION.
Figure 4. ALTER XML SCHEMA COLLECTION Syntax
ALTER XML SCHEMA COLLECTION [ relational_schema. ]sql_identifier ADD
'Schema Component'
|
Table 2. ALTER XML SCHEMA COLLECTION Arguments
| Argument | Description |
|---|
| relational_schema | Identifies the relational schema name. If not specified, the default relational schema is assumed. |
| sql_identifier | Is the SQL identifier for the XML schema collection. |
| ‘Schema Component’ | Is the schema component to insert. |
In Figure 5, we will add a new schema component to the customerSchema.
Figure 5. Sample Usage of ALTER XML SCHEMA COLLECTION
ALTER XML SCHEMA COLLECTION customerSchema ADD
'<schema xmlns="http://www.w3.org/2001/XMLSchema">
<element name="customer" type="string"/>
</schema>'
|
The DROP XML SCHEMA
COLLECTION removes an XML Schema from the database. You cannot drop an
XML schema while it is in use. So if the XML Schema is associated with
an XML-type parameter or column, the table column has to be altered
without specifying the schema. If you try to drop the schema, while in
use, you will receive an error. In the following example, Figure 6, we receive an error when we try to drop the XML Schema, customerSchema, on table XML_Table.
Figure 6. Sample Usage of DROP XML SCHEMA
DROP XML SCHEMA COLLECTION customerSchema
GO
Msg 6328, Level 16, State 1, Line 1
Specified collection 'customerSchema' cannot be dropped because it is used by
object 'dbo.XML_Table'.
|
To drop an XML schema
collection, we will alter the table and remove the schema on the XML
column and then drop the schema, as shown in Figure 7. Refer to Table 3 for a list of arguments and descriptions for the DROP XML SCHEMA COLLECTION.
Table 3. DROP XML SCHEMA COLLECTION Arguments
| Argument | Description |
|---|
| relational_schema | Identifies the relational schema name. If not specified, the default relational schema is assumed. |
| sql_identifier | Is the name of the XML schema collection to drop. |
To properly drop the XML Schema, customerSchema, we need to use the ALTER TABLE statement and then execute a DROP XML SCHEMA COLLECTION. We will use syntax shown in Figure 7 to accomplish this task.
Figure 7. The ALTER TABLE Statement
ALTER TABLE XML_Table ALTER COLUMN customerAddress XML
GO
DROP XML SCHEMA COLLECTION customerSchema
