----- Original Message -----
From: "Jon Anderson" <jon@xxxxxxxxxxxxxxxxxx>
To: "Skip Evans" <skip@xxxxxxxxxxxxxxxxx>
Cc: "PHP-General" <php-general@xxxxxxxxxxxxx>
Sent: Tuesday, January 30, 2007 11:46 PM
Subject: Re: Help wtih a query?
Wrong list. Putting "$sql=..." in there doesn't make it a PHP question.
;-)
Skip Evans wrote:
Is that what the left/right joins do???
Yea. LEFT JOIN will give you NULL entries in the left joined table, so
you'd just have to say WHERE ISNULL(<left joined table>.<some field in
that table>). Of course, you'll need to do the right JOINs in there for
that to work.
Personally, I think that implicit joins are sloppy, so I would suggest
using JOIN with ON or USING...but I suppose that's a preference thing, and
some (all?) might disagree with me.
SQL engines have many techniques to improve the performance of the queries,
with dozens of scholarly papers backing each slight improvement. The more
the SQL engine knows about your intentions, the better chances it has to
apply the best techniques to improve the performance of your query. For a
particular engine, only a subset of all these tricks might be implemented so
little might be gained from writing queries this way instead of that way and
this might change with each version but it still holds that the chances for
better performance improve if you make it clear what you want, eventually.
Satyam
jon
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