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Derived tables are great are are usually faster than correlated subqueries. YOu can also use with staements instead but frankly I find them harder to read and interpret (of course I've been reading derived tables for ten years). One thing to remember is that any derived tblae must be a given an alias or the query will not work.

HLGEM
– HLGEM

2010-10-01 21:40:41 +00:00
Commented Oct 1, 2010 at 21:40
I am speaking from the context of comparably large datasets (though, of course I could still be wrong!). Let's say I wanted to join an aggregate sum within a date range. The derived table is queried before it's assigned to its alias. So it should be faster to say JOIN (select sum(a), b from table where b = c group by b) d than it would be to do JOIN (select sum(a), b from table group by b) d WHERE d.b = c

Matthew
– Matthew

2010-10-04 16:59:12 +00:00
Commented Oct 4, 2010 at 16:59

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