In the earliest versions of MySQL that supported subqueries IIRC you could see different EXPLAINs for what, to me, appeared to be equivalent queries. That is to

In the earliest versions of MySQL that supported subqueries IIRC you could see different EXPLAINs for what, to me, appeared to be equivalent queries. That is to say, I don't believe the query optimizer is/was MySQL's key feature at that time. Then again, it wasn't that long ago (2003) that MySQL 4.0 came out. Before that the subquery format was a syntax error. :/