Let start with the following slightly controversial (over-simplification):
Although not everyone would agree, one of the main reasons for the success of the relational database has been the inclusion of the SQL language. SQL is a set-based declarative language. As opposed to COBOL (or most .NET-based languages for that matter), when you use SQL, you tell the database what data you are looking for, rather than how to obtain that data. The SQL query processor determines the best plan to get the data you want and then retrieves the data for you. As query-processing engines mature, your SQL code will run faster and better (less I/O and CPU) without the developer making changes to the code. What development manager wouldn't be pleased to hear that programs will run faster and more efficiently with no changes to source code because the query engine gets better over time?
So how can we access data in XML format efficiently:
In the early days of XML, imperative programming (navigation through the XML DOM) was all the rage. The XQuery language in general and XQuery inside the database in particular make it possible for the query engine writers to approach the task of optimizing queries against XML. The chances of success are good because these folks have 20 years or so of practical experience optimizing SQL queries against the relational data model. The SQL Server 2005 implementation of XQuery over the built-in XML data type holds the same promise of a declarative language, with optimization through a query engine
The 70-433 exam appears to require basic familarity with the following XML functions:
Table 1. XML Data Type Functions
| Name | Signature | Usage |
| exist | bit = X.exist(string xquery) | Checks for existence of nodes, returns 1 if any output returned from query, otherwise 0 |
| value | scalar = X.value( string xquery, string SQL type) | Returns a SQL scalar value from a query cast to specified SQL data type |
| query | XML = X.query(string xquery) | Returns an XML data type instance from query |
| nodes | X.nodes(string xquery) | Table-value function used for XML to relational decomposition. Returns one row for each node that matches the query. |
| modify | X.modify(string xml-dml) | A mutator method that changes the XML value in place |
The article goes on to detail how this mapped and represented in internal SQL Server database objects.. interesting stuff.
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