Iterate over sequences of items.
A FLWOR expression iterates over a sequence of items and
binds variables that can be used in the scope of the current expression. If the
item sequence is empty, the result of the FLWOR expression is an empty
sequence. A FLWOR expression consists of one or more for and
let clauses in any combination, followed by an optional
where clause, an optional order by clause and a
return clause. Briefly, these clauses are interpreted as
follows:
A for clause binds one or more variables to each value of
the result of the following expression.
A let clause binds one or more variables to the complete
result of the expression.
A where clause retains only those intermediate results
that satisfy the following condition.
An order by clause can reorder the intermediate
results.
A return clause evaluates the following expression and
returns the result.
Retrieve a list of book titles along with a current list number:
for $a at $i in input()/bib/book
return (<BookNo>{ $i }</BookNo>, $a/title)
A positional variable is used for creating the current list number, see
the description of the ForClause for details.
Since a for clause creates tuples of variable bindings
from the Cartesian product of the sequences that the expressions evaluate to, a
cross join is straightforward:
for $i in (1, 2), $j in (3, 4)
return
<tuple>
<i>{ $i }</i>
<j>{ $j }</j>
</tuple>
In SQL contexts, this concept corresponds to the CROSS
JOIN expression.
Select all books for which a review exists, with all authors, title and the review text:
for $b in input()/bib/book,
$a in input()/reviews/entry
where $b/title = $a/title
order by $b/title
return
<book>
{ $b/author }
{ $b/title }
{ $a/review }
</book>
This FLWOR expression is an example of an equijoin: from
both doctypes bib and
reviews only those tuples are retained that satisfy
the equality condition in the where clause. The tuples are then
sorted according to the book title in ascending order. See also the
result in
the XQuery 4 User Guide. In SQL contexts, this concept
corresponds to the JOIN ... ON expression.
A FLWOR expression can often simply be a replacement for a path expression using predicates (filter):
input()/bib/book[author/last = "Stevens"]
can also be written as:
for $a in input()/bib/book where $a/author/last = "Stevens" return $a
Although this query expression is longer than the first one, it generally performs better and it is recommended to use FLWOR expressions instead of path expressions with filters. See also the section Efficient Queries: XQuery in the Performance Guide.
The following construct(s) refer to this construct:
This construct is also related to the following construct(s):
ForClause
|
LetClause
|
OrderByClause
|
WhereClause
|