It's a good question, but I think you've got yourself into a bit of a muddle. I explain a SQL join as "First, all of the records are matched with all of the records (a cartesian join)." "Second, based on your selection criteria, any records that don't match the criteria are removed". So, suppose t1.CIN values are a, c, e and t2.CIN values are b, c, e. After step one, you'll have 9 records with keys a, b; a, c; a, e; c, b; c, c; c, e; e, b; e, c; e, e. Now, step 2, follow the selection criteria. This will remove records c, c and e, e leaving you with 7 records.
Tom
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