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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