Matches in a database.
tensai at zmonkey.org
Tue Jun 28 16:00:20 MDT 2005
On Tue, 2005-06-28 at 15:25 -0600, Matthew Walker wrote:
> Hmmm. I need to learn to use HAVING. I'm self taught, and have always
> just used WHERE. Thanks for showing us how to do this.
There's a difference although there's some overlap. WHERE operates only
on single rows and occurs before GROUP BY operations. HAVING takes
effect after GROUP BY and can thus work on aggregates. So you couldn't
say "WHERE COUNT(col) > 3" but you could use a HAVING. Most WHERE
clauses can move into a HAVING, but I'm sure it's much faster to do
table joins before GROUP functions and databases may not optimize them
the same way.
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