Tamino XQuery User Guide - Software AG Documentation

FLWOR Expressions
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If there are for and let clauses in the FLWOR expression then the variable bindings created by
the let clause are added to the tuples created by the for clause.
The where Clause
By using the where clause you can define conditions that previously generated tuples must satisfy.
If the condition is met, the tuple is retained, if not, the tuple is discarded. It depends on the effective
boolean value of the expression in the where clause, whether tuples will be retained or not. For
the remaining tuples all variable bindings are still valid so that they can be used in the return
clause.
A where clause can simply act as a filter and then represents a more verbose form of a path expression
using a predicate. The query expressions
input()/bib/book[@year < 2000]/title
and
for $a in input()/bib/book
where $a/@year < 2000
return $a/title
are equivalent: Both return the titles of books that have been published before 2000. They differ
in that the path expression additionally performs sorting in document order and elimination of
duplicates according to node identity. Therefore it is not necessary to use the FLWOR expression
here.
But you can use where clauses for more than defining predicates in path expressions. For example,
they are necessary in defining a join criterion:
for $b in input()/bib/book,
$a in input()/reviews/entry
where $b/title = $a/title
return
{ $b/author }
{ $b/title }
{ $a/review }
See also the section Joining for a short explanation of this example.
XQuery User Guide
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