MySQL conditional join using the same table - mysql

Question

I want to do the following using a LEFT JOIN (please do not suggest UNION ALL)
SELECT o.*, s.col1, s.col2 FROM order o
INNER JOIN user u ON o.user_id = u.id
IF o.date less than '2011-01-01'
JOIN subscribe s ON u.key = s.key
ELSE
JOIN subscribe s ON u.email = s.email
END IF;
I used the following but can't test it.
SELECT o.*, COALESCE(s1.col1,s2.col1) AS
col1, COALESCE(s1.col2, s2.col2) AS col2
FROM order o INNER JOIN user u ON o.user_id = u.id
LEFT JOIN subscribe s1 ON
(u.key LIKE (CASE o.date >= '2011-01-01 00:00:00'
WHEN TRUE THEN s1.key ELSE NULL END))
LEFT JOIN subscribe s2 ON (u.email LIKE (CASE o.date <
'2011-01-01 00:00:00' WHEN TRUE THEN s.email
ELSE NULL END));
Please correct me if I am wrong.
Thanks.

Answer 1

You don't need to JOIN twice, do it like this:
SELECT o.*, s.col1, s.col2
FROM order o INNER JOIN user u ON o.user_id = u.id
LEFT JOIN subscribe s ON( ( o.date < '2011-01-01' AND u.key = s.key ) OR
( o.date >= '2011-01-01' AND u.email = s.email ) )
And here is a solution that will not do a full table scan if you have indexes on subscribe.key and subscribe.email:
SELECT * FROM
(
( SELECT 0 AS mode, o.date AS odate, o.*, s.col1, s.col2
FROM order o INNER JOIN user u ON o.user_id = u.id
LEFT JOIN subscribe s ON( u.key = s.key ) )
UNION
( SELECT 1 AS mode, o.date AS odate, o.*, s.col1, s.col2
FROM order o INNER JOIN user u ON o.user_id = u.id
LEFT JOIN subscribe s ON( u.email = s.email ) )
)
WHERE ( odate < '2011-01-01' AND mode = 0 ) OR
( odate >= '2011-01-01' AND mode = 1 )