INNER JOIN and , (comma) are
semantically equivalent in the absence
of a join condition: both produce a
Cartesian product between the
specified tables (that is, each and
every row in the first table is joined
to each and every row in the second
table).
However, the precedence of the comma
operator is less than of INNER JOIN,
CROSS JOIN, LEFT JOIN, and so on. If
you mix comma joins with the other
join types when there is a join
condition, an error of the form
Unknown column 'col_name' in 'on
clause' may occur. Information about
dealing with this problem is given
later in this section.
From:
http://dev.mysql.com/doc/refman/5.0/en/join.html
Hope this helps
