Question

I have a database that contains messages. The messages are stored in one table, the user information is stored in another. In the message table, there is an author_id column which represents the user_id of the author from the user table, there are all the message columns, and there is a to_address which represents a concatenation of “u_” + user_id from the user table. Is there any that I can join these two tables, so that it display the username instead of ID in BOTH the author_id AND to_address.

I’ve tried

SELECT  username, ..., username
  FROM msgs
    INNER JOIN users
      ON user_id=author_id AND concat("u_",user_id)=to_address;

with obvious error
I’ve tried using subqueries such as

SELECT
  ( SELECT username 
      FROM users
        INNER JOIN msgs
          ON user_id=author_id
  ) AS "From",
  ( SELECT username
      FROM users
        INNER JOIN msgs
          ON CONCAT("u_",user_id)=to_address
  ) AS "To",
  ( SELECT timestamp(message_time) FROM msgs
  ) AS "Sent",
  ( SELECT message_subject FROM msgs
  ) AS "Subject",
  ( SELECT message_text AS "Message" FROM msgs
  ) AS "Message"

and got “Subquery returns more than 1 row”. Is there any way that I can do this successfully?

Answer 1

It sounds like you want something like this:

SELECT
    from_user.username AS "From",
    to_user.username AS "To",
    timestamp(msgs.message_time) AS "Sent",
    msgs.message_subject AS "Subject",
    msgs.message_text AS "Message"

FROM msgs

INNER JOIN users AS from_user
ON msgs.author_id = from_user.user_id

INNER JOIN users AS to_user
ON msgs.to_address = CONCAT("u_", to_user.user_id);

Basically, you join the users table to the msgs table twice, giving each instance of the table a different name and a different join condition. Then you can pick a specific column out of a specific instance of the users table.