FLWOR Expressions

FLWOR expressions, also known simply as FLWORs, are used for queries that are more complex. In addition to allowing more readable and structured selections, they allow functionality such as joining data from multiple sources, constructing new elements and attributes, evaluating functions on intermediate values, and sorting results.

FLWOR (pronounced "flower"), stands for "for, let, where, order by, return," the keywords that are used in the expression. Example 6-1 shows a FLWOR that is equivalent to the second path expression from the previous section.

Example 6-1. FLWOR

for $prod in doc("catalog.xml")//product
let $prodDept := $prod/@dept
where $prodDept = "ACC" or $prodDept = "WMN"
return $prod/name

Of course, this is far more verbose, and for such a simple example, the path expression is preferable. However, this example is useful as an illustration before moving on to examples that are more complex. As you can see, the FLWOR is made up of several parts:

for

This clause sets up an iteration through the product elements returned by the path expression. The variable $prod is bound, in turn, to each product in the sequence. The rest of the FLWOR is evaluated once for each product, in this case, four times.

let

This clause binds the $prodDept variable to the value of the dept attribute.

where

This clause selects elements whose dept attribute is equal to ACC or WMN.

return

This clause returns the name child of each of the three product elements that pass the where ...

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