SELECT x, ... FROM ( SELECT y, ... FROM ... ) AS a JOIN tbl ON tbl.x = a.y WHERE ...
This will evaluate the subquery into a temp table, then
JOIN that to
Prior to 5.6, there could not be an index on the temp table. So, this was potentially very inefficient:
SELECT ... FROM ( SELECT y, ... FROM ... ) AS a JOIN ( SELECT x, ... FROM ... ) AS b ON b.x = a.y WHERE ...
With 5.6, the optimizer figures out the best index and creates it on the fly. (This has some overhead, so it is still not 'perfect'.)
Another common paradigm is to have a subquery to initialize something:
SELECT @n := @n + 1, ... FROM ( SELECT @n := 0 ) AS initialize JOIN the_real_table ORDER BY ...
(Note: this is technically a
CROSS JOIN (Cartesian product), as indicated by the lack of
ON. However it is efficient because the subquery returns only one row that has to be matched to the n rows in