It depends very heavily on which RDBMS you use as to what ends up optimized. It's a relatively well trafficked subject, a simple google search turns up quite a

It depends very heavily on which RDBMS you use as to what ends up optimized. It's a relatively well trafficked subject, a simple google search turns up quite a few results, for example: http://www.mysqldbahelp.com/2010/03/subselect-vs-join-performanace.html http://www.mysqldbahelp.com/2010/03/subselect-vs-join-perfor... Disclaimer: I didn't validate his results ;)