use Perl

SELECT ...
FROM a, b, c
WHERE $some_condition_a OR $some_other_condition_b
AND a.foo = b.foo
AND b.bar = c.bar

The problem is that the joining expressions at the end were and'ed only up to the OR, because OR is lower precedence. So, he was getting a full outer join (full cross product) of the three large tables, and futzing memory in the process.

What he should have written was:

SELECT ...
FROM a, b, c
WHERE ($some_condition_a OR $some_other_condition_b)
AND a.foo = b.foo
AND b.bar = c.bar

so that the OR doesn't beat up the AND.

But then I got to thinking... with Real Databases (like PostgreSQL and Oracle) that speak full SQL92, you can always specify the join conditions for the tables up in the SELECT clause:

SELECT ...
FROM a NATURAL JOIN b NATURAL JOIN c
WHERE ($some_condition_a OR $some_other_condition_b)

and now we've clearly separated the expressions required to hook the tables together from the expressions selecting our items of interest. (In fact, the parens are no longer required there.)

Through a combination of NATURAL JOIN, JOIN USING (columns), and JOIN ON (condition), we can always do this. I'm making a resolution to myself to never write a join condition in the WHERE clause, ever again.

And yes, although MySQL halfway supports this (only in some kinds of joins), it also discourages their use. I wonder why.