"CHAN Chor Ling Catherine (CSC)" wrote:
>
> Hi Gurus,
>
> My senior DBA always tell us that the "not in" command sucks and we are all
> encourage to use the select count(*). SQL A is greatly frowned upon and SQL
> B will be the best.
>
> SQL A :
> SELECT col1,col2
> FROM Table_1
> WHERE (col1,col2) NOT IN (SELECT col3,col4
> FROM Table_2
> WHERE col3 = col1
> AND col4 = col2);
> SQL B :
> SELECT col1,col2
> FROM Table_1 A
> WHERE (0=(SELECT COUNT(*) FROM Table_2 b WHERE b.col3=a.col1 AND
> b.col4=a.col2));
>
> Qn : Is it true ? Could someone shed some light ? Please advise. Thanks.
>
> Regds,
> Catherine
>
Larry has pointed to me off-list that your 'SQL A' query is indeed
correlated - totally unusual for a 'NOT IN' and, in your case, such a
case for disaster (couldn't return anything) that I presume that you
typed it as fast as I read it initially?
Being as lazy as he is :-) here is from Larry's message :
> Also, point her towards Metalink note 28934.1. It contains a good
> discussion. But I don't agree with the final conclusion to always use NOT
> EXISTS even though a NOT IN using a HASH AJ is sometimes much better. The
> only reason for that recommendation was their fear that many folks don't
> understand how a NOT IN handles nulls in the results set (returns no rows)
> differently than a NOT EXISTS. A good developer should know the difference.
>
HTH,
Stephane Faroult
Oriole Software
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