具有内部联接和子查询的SQL

Question

我有以下查询：

SELECT * FROM thread where sender = 135 or receiver = 135 order by updatedttm desc

上面的查询返回正确的记录，其中包含senderid和receiveid。 用户表中存在senderid和receiveid。 我想通过将用户表加入上述查询来获取senderid和Receiverid的名称。

Answer 1

您可以两次join用户表，一次使用发送者ID和接收者ID并获得名称。

SELECT t.*,usend.user_name sender_name, urec.user_name receiver_name
FROM thread t
JOIN users usend on usend.user_id = t.sender
JOIN users urec on urec.user_id = t.receiver
where sender = 135 or receiver = 135
order by updatedttm desc

Answer 2

我将使用LEFT JOIN因为即使发送者或接收者被删除，它也会显示您的线程。

SELECT *,
    sender.username AS sender_name,
    receiver.username AS receiver_name
FROM thread AS t
LEFT JOIN users AS sender ON t.sender = sender.user_id
LEFT JOIN users AS receiver ON t.sender = receiver.user_id
WHERE sender = 135 OR receiver = 135 
ORDER BY 
updatedttm DESC