I had the same problem. I was very frustrating with it. Maybe this is not answering the question, but I just want to share my error experience, and there may be others who suffered like me. Evidently it was just my low accuracy.
I had this:
SELECT t_comment.username,a.email FROM t_comment
LEFT JOIN (
SELECT username,email FROM t_un
) a
ON t_comment.username,a.email
which is supposed to be like this:
SELECT t_comment.username,a.email FROM t_comment
LEFT JOIN (
SELECT username,email FROM t_un
) a
ON t_comment.username=a.username
Then my problem was resolved on that day, I'd been struggled in two hours, just for this issue.