Question

我需要确定以下问题:

SELECT *
FROM message_treads
LEFT JOIN usernames ON usernames.uid=message_treads.uid_1
LEFT JOIN usernames ON usernames.uid=message_treads.uid_2

I know i should use aliases, but i am not sure on how to do it here. I know how to do it when there is only one left join of one table, but how to to it when there are more than one?

Answer 1

你们是正确的,你们需要使用诸如:

SELECT *
FROM message_treads
LEFT JOIN usernames uid1 ON uid1.uid=message_treads.uid_1
LEFT JOIN usernames uid2 ON uid2.uid=message_treads.uid_2

Answer 2

你可以这样做:

SELECT *
FROM message_treads
LEFT JOIN usersnames ON usernames.uid IN (message_treads.uid_1, message_tread.uid_2)

具体来说,你就是这样:

SELECT *
FROM message_treads
LEFT JOIN usernames AS user1 ON user1.uid = message_treads.uid_1
LEFT JOIN usernames AS user2 ON user2.uid = message_treads.uid_2

但是,这很可能不是你想要的,因为这一差价从翻一番的用户名表中退回了两组。上述第一版将只退回一组用户名栏。

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