EXISTS, NOT EXISTS
You can define subquery in cbQL. You can use each field in the subquery. The exists field checks whether there is a relation between the result of the subquery and the result of the query.
For example:
tracker.id = 7 and exists(tracker.id = 9)
The result contain only elements which are in the tracker, where the id = 7 and which have a relation to or from items which are in the tracker where id = 9