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XQuery and SQL/XML Views

Some people seem to believe that the purpose of XQuery is largely the same as that of SQL/XML - to allow XML structures to be created from relational data. Although XQuery is useful for this task, it has relatively few advantages over SQL/XML when this is all that is required. The reason for this is simple: SQL is a language designed for handling SQL data sources, and it does that very well. Adding XML publishing functions to SQL is a simple way to let it create XML. However, it is interesting to note that the SQL/XML views of relational tables have a very constrained structure, and XQuery performed on such views is generally quite similar to the equivalent SQL/XML.

For instance, let's write an XQuery equivalent to the last SQL/XML query we used. This query will operate on a SQL/XML view of the relational tables. The Projects table is represented as follows:

The Customers table is represented as follows:

We want to rename these elements and create a representation that shows customers together with their projects. The output should look like this:

Here is an XQuery that creates the desired output:

Let's compare this XQuery to the SQL/XML query from a prior section:

In this example, as in most such examples, it is hard to argue that either solution is particularly superior to the other. Either SQL/XML or XQuery handle such tasks quite well. The real strength of XQuery is in the ability to easily process XML, whether or not relational data is being processed, including the XML that is frequently stored in columns of relational databases and the XML of web messages. Since XQuery also works well on SQL/XML views of relational data, it is particularly useful when both XML data and relational data must be used in processing. This is explored in the next section.