Overview

With relational database management systems, it is common practice to create a view of a database and then query that view using SQL. XDR schemas perform the similar function of creating XML views of relational data, which can then be queried via XPath expressions.

As we saw in Chapter 1, XML schemas describe XML document structure and are capable of placing constraints on the data in the document. SQL Server 2000 uses the XML-Data Reduced language to create schemas. This language is similar but only similar to the W3C specification for a schema language, which is located at http://www.w3.org/TR/2000/CR-xmlschema-0-20001024/. Although the two methods accomplish basically the same thing, the languages are very, very different. Now let's take a look at XDR schemas and the XDR language as a whole.