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FILTER statement can be used to restrict the results to elements that
match an arbitrary logical condition.
expression must be a condition that evaluates to either false or true. If
the condition result is false, the current element is skipped, so it will not be
processed further and not be part of the result. If the condition is true, the
current element is not skipped and can be further processed.
See Operators for a list of comparison operators, logical
operators etc. that you can use in conditions.
FOR u IN users FILTER u.active == true && u.age < 39 RETURN u
It is allowed to specify multiple
FILTER statements in a query, even in
the same block. If multiple
FILTER statements are used, their results will be
combined with a logical AND, meaning all filter conditions must be true to
include an element.
FOR u IN users FILTER u.active == true FILTER u.age < 39 RETURN u
In the above example, all array elements of users that have an attribute
active with value true and that have an attribute age with a value less
than 39 (including null ones) will be included in the result. All other
elements of users will be skipped and not be included in the result produced
RETURN. You may refer to the chapter Accessing Data from Collections
for a description of the impact of non-existent or null attributes.
Order of operations
Note that the positions of
FILTER statements can influence the result of a query.
There are 16 active users in the test data
FOR u IN users FILTER u.active == true RETURN u
We can limit the result set to 5 users at most:
FOR u IN users FILTER u.active == true LIMIT 5 RETURN u
This may return the user documents of Jim, Diego, Anthony, Michael and Chloe for
instance. Which ones are returned is undefined, since there is no
to ensure a particular order. If we add a second
FILTER statement to only return
FOR u IN users FILTER u.active == true LIMIT 5 FILTER u.gender == "f" RETURN u
… it might just return the Chloe document, because the
LIMIT is applied before
FILTER. No more than 5 documents arrive at the second
and not all of them fulfill the gender criterion, eventhough there are more than
5 active female users in the collection. A more deterministic result can be achieved
by adding a
FOR u IN users FILTER u.active == true SORT u.age ASC LIMIT 5 FILTER u.gender == "f" RETURN u
This will return the users Mariah and Mary. If sorted by age in DESC order,
then the Sophia, Emma and Madison documents are returned. A
FILTER after a
LIMIT is not very common however, and you probably want such a query instead:
FOR u IN users FILTER u.active == true AND u.gender == "f" SORT u.age ASC LIMIT 5 RETURN u
The significance of where
FILTER blocks are placed allows that this single
keyword can assume the roles of two SQL keywords, WHERE as well as HAVING.
FILTER thus works with
COLLECT aggregates the same as with any other
intermediate result, document attribute etc.