XML TYPE FUNCTIONS Although this similar to a

XML TYPE FUNCTIONS Although this similar to a normal SQL primary key and index creation statement with an extra XML keyword, the actual effect of the statement is much different from creating a SQL Server index. What you are creating in the case of an XML column is an index over the internal representation or structure of the column whose purpose is to optimize XQuery queries rather than SQL queries. Remember that the SQL comparison operators cannot be used on an XML column. However, because the index contains the (SQL) primary key of the table, it can assist in queries that use XQuery criteria and a primary key value in a SQL WHERE clause. Though the internal representation of the XML index is an internal implementation detail, suffice it to say that creating such an index will not help optimize queries that cast or convert the XML data to character types first. Because the XML index is not a normal SQL index, some limitations apply to these indexes. You cannot create an XML composite index that is, an index on more than one XMLcolumn or an XMLcolumn and a non-XMLcolumn. You cannot create an XML index as a clustered index or use it as a partitioning criterion. In addition, because all XML indexes and SQL indexes share the same value space in a database, you cannot create an XML index (of any kind) and a SQL index with the same index name, or two different kinds of XML index with the same name. Although an XML type can also be used with full-text search, this is outside of the scope of this book. XML Type Functions In addition to being used as a table or view column, variable, or parameter in its entirety, the XML data type contains a variety of type-specific methods. These are invoked by using variable.method syntax, similar to methods of .NET user-defined types. Although the syntax is the same, it should be pointed out that the XMLdata type is not implemented as a .NET user-defined type. The implementation of the XML type is proprietary, private, and not open to programmers to extend or refine. The XML type functions encompass a few different groups of functionality. Determining if a node or nodes exist that satisfy a given XQuery expression (exist)

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XML IN THE DATABASE: THE XML DATA TYPE

XML IN THE DATABASE: THE XML DATA TYPE Creating an Index on an XML Column You can create indexes on an XML column, using approximately the same syntax that you use for a SQL index. Four kinds of XML indexes can be created. In order to create any kind of XML index, the table must have an ordinary SQL data type primary key column. You must first create the primary index or node table. This index associates each node with the SQL key column and is useful for ad hoc queries. You can create an XML index over only the document structure, using the FOR PATH keyword. This is similar to the concept of creating a key in XSLT; this type of index helps in XQuery path statements. You can also create an index only over the values of the elements and attributes in the XMLdata type column with the FOR VALUE keyword. This type of index can help in XQuery content searches. The FOR PROPERTY keyword creates an index that is most usable when your XML consists of a shallow hierarchy with many elements or attributes that are really name/value pairs. Additional XML index types may be defined in the future. If you create an XML index on the entire document or column, you cannot index subsets of the document. The syntax is shown in Listing 8-6. Listing 8-6: Creating XML Indexes CREATE TABLE xml_tab( the_id INTEGER PRIMARY_KEY IDENTITY, xml_col XML) GO CREATE PRIMARY XML INDEX xmlidx1 ON xml_tab(xml_col) GO structural index CREATE XML INDEX xmls1 ON xml_tab(xml_col) USING XML INDEX xmlidx1 FOR PATH GO property index CREATE XML INDEX xmlp1 ON xml_tab(xml_col) USING XML INDEX xmlidx1 FOR PROPERTY GO value index CREATE XML INDEX xmlv1 ON xml_tab(xml_col) USING XML INDEX xmlidx1 FOR VALUE GO

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TYPED AND UNTYPED XML Notice that the

TYPED AND UNTYPED XML Notice that the simpleType treeAge2 in the namespace http:// example.org/Trees2 is derived by restriction from the base type t2:treeAge in a different namespace. We could also have defined both schemas in the collection with a single CREATE XML SCHEMA COLLECTION DDL statement. An XML schema collection is dropped from the database like any other SQL Server object and is subject to the same constraints; for example, you may not drop an XML schema collection if it is being used to type an XML data type column. this would fail if the XML schema collection was used to type an XML data type column DROP XML SCHEMA COLLECTION mycoll Security for Schema Collections and Strongly Typed Instances Security for XML schema collections and strongly typed instances is applied on the database objects and is analogous to security on native SQL Server data. XML schema collections are scoped to the SQL schema within a database, so you can permit users or roles to define XML schemas. GRANT CREATE XML SCHEMA COLLECTION TO public Once a specific XML schema collection is cataloged, permissions must be granted to reference the schema or use strongly typed columns or parameters. The syntax looks somewhat like the syntax for a .NET procedure or UDT. GRANT REFERENCES ON XML SCHEMA COLLECTION mycoll TO FRED The permissions that can be granted on a specific schema collection are as follows: REFERENCES Gives permission to define tables and views that reference a schema collection EXECUTE Gives permission to use strongly typed columns, parameters, or variables that refer to a given schema collection

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XML IN THE DATABASE: THE XML DATA TYPE

XML IN THE DATABASE: THE XML DATA TYPE ) GO XML schema collections are tied to a specific SQL schema within a specific database; they are first-class SQL Server database objects that can be referenced by a one-, two-, or three-part name by users with the appropriate permission. Because many XML documents use types from multiple XML namespaces, an XML schema collection can contain multiple unrelated schemas. In addition, many XML schemas import types from other schemas; you can use the XML schema statement to import another XML schema namespace that you use in a second schema definition for a different namespace. In addition to defining all the schemas in a collection by using the CREATE XML SCHEMA COLLECTION DDL statement, you can add more schemas to a collection after it s been created using ALTERXMLSCHEMACOLLECTION. For example, if we first define the following XML schema collection: CREATE XML SCHEMA COLLECTION mytrees AS it is permissible to import that schema definition into another schema definition for a different namespace, as the following code shows. ALTER XML SCHEMA COLLECTION mytrees ADD

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TYPED AND UNTYPED XML xml_schema_namespace(), as shown in

TYPED AND UNTYPED XML xml_schema_namespace(), as shown in the following example, comments and schema annotations are not recovered. this returns a single XML schema document for the http://invoice namespace if it occurs in the XML schema collection mycoll DECLARE @s XML SELECT @s = xml_schema_namespace( N dbo , N mycoll , N http://invoice ) PRINT CONVERT(varchar(max), @s) GO this returns all of the XML schema documents in the mycoll XML schema collection multiple schema documents are separated by a space DECLARE @s2 XML SELECT @s2 = xml_schema_namespace(N dbo , N mycoll ) PRINT CONVERT(varchar(max), @s2) GO The exact signature of xml_schema_namespaceis: XML_SCHEMA_NAMESPACE (relational_schema, xml_schema_collection_name[,namespace]) where relational_schema is the relational schema that contains the collection. It returns a schema document (as an XML datatype instance) representing the content of the XML schema namespaces associated with the SQL XML schema collection identifier. If you need to keep track of an original schema in its exact text format, you should store it separately. A convenient way to keep track of your schemas is to insert them into a table with a NVARCHAR(max)column. CREATE TABLE xml_schema_save_tab ( the_id INTEGER PRIMARY KEY, xml_schema_col NVARCHAR(max)) go INSERT INTO xml_schema_save_tab VALUES(1, N < ! this schema defines a single data type, personAge, and I want to save this comment as well as the schema >

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XML IN THE DATABASE: THE XML DATA TYPE

XML IN THE DATABASE: THE XML DATA TYPE this works with either table, a single root element INSERT INTO person_docs VALUES( 11 ) INSERT INTO person_content VALUES( 11 ) this fails, more than one root element INSERT INTO person_docs VALUES( 5 5 ) GO this works because it s a valid fragment INSERT INTO person_content VALUES( 5 5 ) GO XML variables can also be schema-validated by declaring them with a name of an already-defined schema collection. this document or fragment must correspond to this schema collection DECLARE @x XML(accountingcoll) input is validated here SET @x = 4321 10753 987-65 5 There are three ways to specify the type of an XML element in an XML document. You can define an XML namespace prefix for a particular name- space and use the prefix on the element in question, use a default XML namespace definition, or use xsi:typeto specify the data type. Management of XML Schemas and Schema Collections When an XML schema collection is stored in SQL Server, its schemas are not stored directly as XML documents. Instead, they are shredded into a proprietary format that is useful for optimizing schema validation. Although you can reconstruct your XML schemas using the system function

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TYPED AND UNTYPED XML XQuery engine because you

TYPED AND UNTYPED XML XQuery engine because you are using typed data in the query. It also allows the XQuery engine to know the data type of its intermediate and final results. If the XML data is not strongly typed, XQuery treats everything as a weakly typed string called xdt:untypedAtomic. The integrity checking for a typed XML column happens each time a new value is set into it. This would occur when you are inserting or updating the column into a table. As an example, creating the following table and adding some rows to it will cause each instance of the XMLdata type to be schema-validated at insert time. CREATE TABLE person_tab( id INT IDENTITY PRIMARY KEY, the person column can only contain infoset items that are defined in the schema collection defined above person XML(peoplecoll)) GO this works, person between 0 and 30 years old INSERT INTO person_tab VALUES( 11 ) so does this, using the no-namespace schema INSERT INTO person_tab VALUES( 75 ) this insert fails INSERT INTO person_tab VALUES( 31 ) You can precede your schema collection identifier with the keywords DOCUMENTor CONTENT. If you do not use one of these keywords, the default is equivalent to specifying CONTENT. If DOCUMENT is specified, the column can only contain XML documents (a document is defined as having a single root element), but if you specify CONTENT, the column can contain documents or fragments, as long as all the elements are schema-valid. Here s an example that illustrates the difference. CREATE TABLE person_docs( id INT IDENTITY primary key, person XML(DOCUMENT peoplecoll)) GO CREATE TABLE person_content( id INT IDENTITY PRIMARY KEY, person XML(CONTENT peoplecoll)) GO

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