MySQL version 4.1 was quite revolutionary. The main reason for that was support for sub-queries.1
However since then MySQL users were rather discouraged to use that functionality, basically due to the implementation’s poor performance and forced to build complicated queries based on joins rather than on subqueries.
Of course you can do some effort to optimize your subquery with sometimes very good results2. Not always it’s easy or even possible if you can’t change the code though.
You’d say it’s[Read more...]