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Re: INNER JOIN (WHERE) vs LEFT JOIN
Posted by: Sergey Petrunya
Date: November 03, 2005 05:26AM

> Secondly, compare the two SQLs:
> SELECT * FROM Table1, Table2 WHERE Table1.columnA
> = Table2.columnB;
> and
> SELECT * FROM Table1 LEFT JOIN Table2 ON
> Table1.columnA = Table2.columnB;
>
> Which would be executed faster? I understand the 2
> SQLs have different function. LEFT JOIN should
> used for scenario if not all record of
> Table1.columnA exist in Table2.columnB. But
> assuming it does, which query would be faster?

It depends on if there are indexes on columnA or columnB and which index is better etc. In general, the optimizer will have more options for executing inner join than for outer join, so inner join has a chance of being executed faster.

The execution of nested joins is described in details here:
http://dev.mysql.com/doc/refman/4.1/en/left-join-optimization.html
http://dev.mysql.com/doc/refman/5.0/en/nested-joins.html

Sergey Petrunia, Software Developer
MySQL AB, www.mysql.com
My blog: http://s.petrunia.net/blog

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