[tremon]

There is a semantic difference, in the context of handling NULLs: EXISTS only checks the size of the result set, so even a subquery that returns (NULL, NULL, NULL) will satisfy the EXISTS condition. IN, on the other hand, performs data equality checks on the results, so NULL rows will never yield a positive match.

Usually, this doesn't make a difference as an EXISTS check is mostly used on the primary key (or business key) of a table, which is (hopefully) non-nullable. But it can sometimes give surprising results when using EXISTS on a nullable column, or for worse results, NOT IN.