具有内部联接和子查询的SQL

Question

I have following query:

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

The above query returns correct record which has a senderid and receiverid. The senderid and receiverid are both present in user table. I want to get names of both senderid and receiverid by joining user table to above query.

Answer 1

You can join the user table twice, once with the sender id and the receiver id and get the name.

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

I would go with LEFT JOIN as it will display your thread even if the sender or the receiver gets deleted.

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