The SQL XML data type is a data type that allows for the storage and manipulation of XML data within a relational database management system (RDBMS). XML, or Extensible Markup Language, is a format for representing and exchanging structured data on the web. By using the SQL XML data type, developers can store and retrieve XML data in a format that is easily accessible and searchable using SQL queries.

In SQL Server, the XML data type is a native data type that was introduced in SQL Server 2005. It can be used to store XML documents, XML fragments, or even XML namespaces. To declare a column as an XML data type, the XML keyword is used in the CREATE TABLE statement. For example:

   myXMLColumn XML

Once the XML data is stored in the column, it can be manipulated using XML functions in SQL. Some of the commonly used XML functions in SQL Server include:

QUERY(): Returns a subset of the XML data that matches an XQuery expression.
VALUE(): Extracts a single value from an XML instance.
MODIFY(): Inserts, updates, or deletes XML data in the column.

The SQL XML data type is particularly useful for applications that require the storage and manipulation of complex and hierarchical data structures. It provides a convenient way to store and retrieve XML data in a relational database, without the need for a separate XML storage system. However, it is important to note that working with XML data can be more complex than working with other data types, and it requires a good understanding of the XML format and XML-specific SQL functions.