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Spanning Sources: XQuery, Web Messages, and Databases

XQuery, when combined with a SQL/XML view of a relational database, is extremely good for processing XML together with relational data. This is a very common requirement in many environments, including web message processing environments. To illustrate this, we will use Example 1 from the SOAP Primer. The task is as follows: an incoming message requests a flight to Los Angeles departing from New York as follows:

According to the SOAP Primer, the proper response is to point out that there are three airports that depart from New York, so that the user can be prompted to pick one. Here is the desired output:

Reading between the lines, we assume that there is a database somewhere that lists the airports for each city. The SQL/XML view of the airports table might look like this:

We will assume that when there is only one airport for a city, the output should simply list that city, and that an error should be raised if there is no airport for a given city. The following XQuery handles all three of these cases:

Note that this code operates at a level very close to the application domain, rather than navigating XML documents and converting from XML to appropriate types in the host language. XML data sources and relational data sources are treated in the same way - to the query, they both look like XML documents.