Join mysql tables twice on 2 columns = 1 column - mysql

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.

Answer 2

I think you want to do something like
SELECT msgs.*,
authors.whatever,
addresses.to_address
FROM msgs
JOIN users AS authors ON msgs.author_id = authors.id
JOIN users AS addresses ON msgs.address_id = addresses.id
My query is perhaps imprecise but you can probably see what I'm doing here.
As an aside, I would recommend not abbreviating msgs and using singular table names.

Answer 3

You need two joins as you want to get two separate users:
select f.username, t.username
from msgs m
inner join users f on f.user_id = m.author_id
inner join users t on concat("u_", t.user_id) = m.to_address

Answer 4

This will return the username associated with both the "author_id" and the "to_address", using correlated subqueries, instead of using JOIN. (Using a JOIN is the usual approach, but an approach using a correlated subquery gives you some additional flexibility.
SELECT (SELECT u.username
FROM users u
ON u.user_id = CONCAT("u_",u.user_id) = m.to_address
ORDER BY u.username LIMIT 1
) AS to_username
, (SELECT a.username
FROM users a
ON a.user_id = m.author_id
ORDER BY a.username LIMIT 1
) AS author_username
, m.*
FROM msgs m
NOTE: this differs a bit from an INNER JOIN in that this will return a row from msg when a matching username is not found for the to_address or the author_id.)
NOTE: this assumes that user_id is unique in the users table.
NOTE: if the username column is NOT NULL in the users table, then you can emulate the INNER JOIN, and NOT return a row if a matching username is not found for the author_id or to_address by adding
HAVING to_username IS NOT NULL
AND author_username IS NOT NULL