MySQL: subquery in LEFT JOIN

Question

I have a problem. I want to use subquery tm in LEFT JOIN .. ON

SELECT t.*, 
    (SELECT `uid` FROM `truck_transport` tm WHERE tm.from = t.station ORDER BY RAND() LIMIT 1) as tm 
FROM `truck_trailer` t 
LEFT JOIN `truck_transport` tm2 ON (tm2.uid = tm) ...

If I use subquery in FROM result of rand is always the same.

Sorry for my language :/

Answer 1

try this

SELECT t.*, tm.uid
FROM `truck_trailer` t 
    LEFT JOIN (SELECT `uid` FROM `truck_transport` ORDER BY RAND() LIMIT 1) as tm 
    ON (tm.uid = t.station)

Answer 2

mynawaz写了一个正确的查询。

由于limit 1，您的子查询将始终只返回一个结果。如果您只想要一个结果，请使用JOIN代替LEFT JOIN。因为左连接表总是返回匹配的行，与右侧表的NULL不匹配。