A similar question recently came up on the dbi-users mailing list. Check out the discussion in the archives, starting at Checking for the existence of a certain row.

Quick summary: EXISTS is faster than COUNT(*) if the value exists in the column, especially if the column is indexed, but if the value doesn't exist in the column the query's going to be slow no matter what.

In reply to Re: Comparing an array to a DBI table by chipmunk
in thread Comparing an array to a DBI table by oakbox

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