Hi,

or probably more accurately a few more brains.

I think the following are equivalent:

do something (select, update, delete)
where < some predicates>
and exists ( select 'x' from tableA, tableB
 where < some regular predicates > and < some correlated predicates >
 UNION ALL
 select 'x' from tableC, tableD (could be same as A and/or B)
 where < some regular predicates > and < some correlated predicates >
 )

and

do something (select, update, delete)
where < some predicates>
and ( exists ( select 'x' from tableA, tableB
 where < some regular predicates > and < some correlated predicates > )
 OR exists (
 select 'x' from tableC, tableD (could be same as A and/or B)
 where < some regular predicates > and < some correlated predicates > )
 )

Likewise for not exists:

do something (select, update, delete)
where < some predicates>
and not exists ( select 'x' from tableA, tableB
 where < some regular predicates > and < some correlated predicates >
 UNION ALL
 select 'x' from tableC, tableD (could be same as A and/or B)
 where < some regular predicates > and < some correlated predicates >
 )

vs.

do something (select, update, delete)
where < some predicates>
and not exists ( select 'x' from tableA, tableB
 where < some regular predicates > and < some correlated predicates > )
 AND not exists (
 select 'x' from tableC, tableD (could be same as A and/or B)
 where < some regular predicates > and < some correlated predicates > )

Can someone think of some condition(s), examples where they would not
be equivalent?



Regards

Wolfgang Breitling
Centrex Consulting Corporation
www.centrexcc.com
AIM: WBreitling



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