Sorry, that's not a very good formulation of the question.
I have 2 tables, and I want to retrieve data from records in table
A only if there are corresponding records in B. I don't care what's in the
records in table B: all I want to know is which items in A _have_ records
in B.
If I do:
select distinct A.this, A.that from A, B
where A.key=B.key
order by A.key
then the join produces zillions of records and the query takes ages.
Similarly if I do
select distinct A.this, A.that from A
left join B on A.key=B.key
where B.key is not null
it takes ages.
I guess if I could somehow apply 'limit 1' to the join, so that I
just get any one record from B (I don't care which record: after all I'm
not using data from B) then it would go like greased lightning, but I
can't see how to express that in mySQL SQL. Is this one of those things
that can't be done without sub-selects? Something like:
select A.this A.that from A
where (select count(*) from B where B.key=A.key)>0
(I'm not sure if that's valid SQL, but I hope it conveys my meaning.)
regards,
--
John Stumbles [EMAIL PROTECTED]
I.T. Services Centre, University of Reading http://www.rdg.ac.uk/~visstmbl
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never generalise
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