A query such as:

SELECT pm1.g1.e1, pm1.g1.e2, /*+ MJ */ (select count(*) from pm1.g2 where e1 = pm1.g1.e1) FROM pm1.g1

Where the subquery contains aggregates without a group by will produce a join plan such that a group by is added to account for the projection of the e1 value for the join predicate. However that will change the value from 0 to null for counts from pm1.g2 where there are no matching rows.