XQuery and FLWOR

Of course, XQuery offers a lot more than simple runtime document selection. It's most powerful when you harness what's often called its FLWOR capabilities. FLWOR is an acronym for "for, let, where, order by, return." These are all clauses you can use in your XQuery expressions to get more precise results.For SQL veterans, you should already begin to feel a little more comfortable. WHERE and ORDER BY are both common parts of SQL queries. And for programmers, the term for should look familiar. Here's the brief rundown on what FLWOR clauses do:

• for: You can use for to take a node set and iterate over it. In many ways, for is the assignment of a variable to the current value in a node set, so you can operate on that variable.
• let: You assign values to variable with let, although (as you'll soon see) you won't use let as often as you use the other FLWOR clauses.
• where: where lets you apply selection criteria to a node set, beyond what XPath offers. Of course, in many queries, you'll see that where isn't doing more than XPath; it's just moving the predicate on an XPath to a different place.
• order by: The order by clause doesn't change data, or filter it; it just applies an ordering to a result set, and allows you to sort values based on something other than the location used in the XPath itself.
• return: Using a return clause lets you operate on a node set, but then return (in results) something other than that node set. You might want to select a set, order and filter it, and then return only the child elements of the results; return is the key that makes that possible.