1.2 Retrieved nulls have special logic to handle outer joins

In the course of executing a select query, the following rules are applied:

1. Any selected attribute that is null does not bind its associated variable.
2. Just before returning from the query any select variables that are still free are bound to {null}.

This is so we can handle outer joins. Consider this:

x :: [a-A] *== y :: [a-A]

Assume x.a binds A to a non-null value. If there is no matching row in y, then y.a = null. If variable A was truly shared the query could never succeed. By not binding the variable associated with y.a the query can succeed ( rule 1) and A will be bound to the value in x.a.