When creating query relationships, keep the
following guidelines in mind.
There must be only one “root” query that
has no arrows pointing into it. This is the main query that is
executed. All other queries must be related to the “root” query in
some way.
Circular relationships are not allowed.
For example, if Query A has a relationship to Query B, Query B
can’t have a relationship back to Query A.
You can link one query to another using
more than one column.