SELECT语句在另一个SELECT语句中

Question

I have two tables called messages and users. In the messages table, there's a field which is a foreign key to the users table that is basically the users ID. I am trying to retrieve results from the messages table using a SELECT query, but I want the users username rather than their user ID. This SQL is wrong but I think it gets across the idea of what I'm trying to do:

SELECT (SELECT username FROM `users` WHERE u_id=?), message, sent FROM `messages` WHERE r_id=? AND sent > ?

Basically, I want to use the users ID stored in the messages table to get the users username to be returned AND get results from the messages table in one query.

I think JOINs are the tool for this, but I have very little bar no SQL experience.

Thanks.

Answer 1

You want to JOIN the two tables together, using the common u_id column.

SELECT u.username, m.message, m.sent
    FROM messages m
        INNER JOIN users u
            ON m.u_id = u.u_id 
    WHERE m.r_id = ?
        AND m.sent > ?