Re: Performance impact of filtering in ON vs WHERE clause

[Apachez]

http://dev.mysql.com/doc/refman/5.0/en/join.html

"
The ON conditional is any conditional expression of the form that can be used in a WHERE clause. Generally, you should use the ON clause for conditions that specify how to join tables, and the WHERE clause to restrict which rows you want in the result set.
"

The use of "USING" also points to how joins should be threated...

t1 LEFT JOIN t2 ON (t1.id = t2.id)

is equal to

t1 LEFT JOIN t2 USING (id)