The FILTER statement can be used to restrict the results to elements that match an arbitrary logical condition.
condition 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 by 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 instance:
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 SORT statement to ensure a particular order. If we add a second FILTER statement to only return women...
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 the second FILTER. No more than 5 documents arrive at the second FILTER block, 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 SORT block:
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. AQL's FILTER thus works with COLLECT aggregates the same as with any other intermediate result, document attribute etc.