"There are many times as a database developer and administrator
that you'll want to know what entities are being excluded from a
straight table join.
"For instance, it would be instructive for management to
identify sales people who haven't received any bonuses, as these
might be deemed to be the underperformers. To provide the requestor
with the statistics, you'll have to determine which sales people
are absent from the bonuses table. This is commonly referred to as
a Non-matching query, although it is sometimes called a Subtract or
Set Difference query as well."