I have a particularly slow query due to the vast amount of information being joined together. However I needed to add a where clause in the shape of id in (select id from table).
I want to know if there is any gain from the following, and more pressing, will it even give the desired results.
select a.* from a where a.id in (select id from b where b.id = a.id)
as an alternative to:
select a.* from a where a.id in (select id from b)
Update: MySQL Can't be more specific sorry table a is effectively a join between 7 different tables. use of * is for examples
Edit, b doesn't get selected
Your question was about the difference between these two:
select a.* from a where a.id in (select id from b where b.id = a.id)
select a.* from a where a.id in (select id from b)
The former is a correlated subquery. It may cause MySQL to execute the subquery for each row of a.
The latter is a non-correlated subquery. MySQL should be able to execute it once and cache the results for comparison against each row of a.
I would use the latter.