Re: Help with SQL

It depends on the dataset. If your exception join brings in 5 child
records that match what you're looking for, you are then ignoring 4 records
because what you really wanted was to just know if there was a match to
your primary record. This is also why a "FETCH FIRST ROW ONLY" will perform
better on a column sub-select, even when there is only one record ever
returned, because you've told the optimizer that you're only interested in
one record so it won't look for more than one, which would incidentally
also crash the sub-select.



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Charles Wilt
Sent: Friday, December 20, 2019 10:53 AM
To: Midrange Systems Technical Discussion <midrange-l@xxxxxxxxxxxxxxxxxx>
Subject: Re: Help with SQL

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The query engine is optimized for joins.

Many times, EXISTS or a IN will be re-written as a join.

An exception join, as shown, will not bring try to bring in any data from
the child table.

Charles


On Fri, Dec 20, 2019 at 8:47 AM Darren Strong <darren@xxxxxxxxx> wrote:

I've seen NOT EXISTS and JOINS suggested. My opinion is that a NOT
EXSISTS will perform the best for you. If you don't actually need a
value from the child table, a join brings the possibility of a higher

performance hit vs.

the NOT EXISTS, which is accurately telling the optimizer that you
don't want anything from the secondary table except to know if a
record exists there.

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