Chapter 9. Advanced Queries
Now that you are an expert on the syntax of XQuery expressions, let’s look at some more advanced queries. This chapter describes syntax and techniques for some commonly requested query capabilities. You may have these same requirements for your queries, but even if you don’t, this chapter will show you some creative ways to apply XQuery syntax.
Working with Positions and Sequence Numbers
Determining positions and generating sequence numbers are sometimes challenging to query authors who are accustomed to procedural programming languages. Because XQuery is a declarative rather than a procedural language, it is not possible to use familiar techniques like counters. In addition, the sorting and filtering of results can interfere with sequence numbers. This section describes some techniques for working with positions and sequence numbers.
Adding Sequence Numbers to Results
Suppose you want to return a list of product names preceded by a sequence number. Your first approach might be to use a variable as a counter, as shown in Example 9-1. However, the results are not what you might expect. This is because the
return clause for each iteration is evaluated in parallel rather than sequentially. This means that you cannot make changes to the value of a variable in one iteration, and expect it to affect the next iteration of the
for clause. At the beginning of every iteration, the
$count variable is equal to 0.
Example 9-1. Attempting to use a counter variable