mysql select statement with multiple where/conditions - mysql

Question

I have the following two tables in mysql:
users:
+--------+-----------+
| userId | userName |
+--------+-----------+
| 1 | magnus |
| 2 | fabiano |
| 3 | alexander |
| 4 | veselin |
+--------+-----------+
games:
+--------+---------+---------+
| gameId | userId1 | userId2 |
+--------+---------+---------+
| 1 | 1 | 2 |
| 2 | 1 | 3 |
| 3 | 2 | 3 |
| 4 | 2 | 4 |
+--------+---------+---------+
How can I construct a single query such that I get this below output of say, fabiano's opponents:
output:
+--------+-----------+
| gameId | userName |
+--------+-----------+
| 1 | magnus |
| 3 | alexander |
| 4 | veselin |
+--------+-----------+
Edit1:
This was what I was trying and I wasn't able to get them into a single query:
select fabiano's opponents [select * from games where 2 in (userId1, userId2);]
read each of the rows, and check which of them is fabiano(2), and select the other userId
from the userIds of these opponents, get their name from users table
Edit2:
Inspired by the answers below, I wrote this (they work):
-- NO JOIN
select x.gameId, users.userName from
(
select gameId, userId2 as id from games where userId1=2
UNION
select gameId, userId1 as id from games where userId2=2
) as x, users
where users.userId = id;
-- NO JOIN, NO UNION
select x.gameId, users.userName from (
SELECT g.gameId,
CASE WHEN userId1 = 2
THEN userId2
WHEN userId2 =2
THEN userId1
END AS id
FROM games g) as x, users
where users.userId = id;

Answer 1

You can union the two sets of data together, viz all games where Fabiano is User 1, with all games that he is in the role of User 2:
SELECT x.Opponent
FROM
(
SELECT u.Name AS Opponent
FROM games g
INNER JOIN users u
ON g.userId2 = u.UserId
WHERE g.UserId1 = 2 -- Fabiano
UNION
SELECT u.Name
FROM games g
INNER JOIN users u
ON g.userId1 = u.UserId
WHERE g.UserId2 = 2 -- Fabiano
) AS x;
At this point as assume that Fabiano can't simultaneously both be User1 and User2, as we would need to consider UNION ALL vs UNION DISTINCT :)
This could also be tidied up a bit into:
SELECT x.Opponent
FROM
(
SELECT u.Name AS Opponent, g.UserId1 AS PlayerId
FROM games g
INNER JOIN users u
ON g.userId2 = u.UserId
UNION
SELECT u.Name, g.UserId2 AS PlayerId
FROM games g
INNER JOIN users u
ON g.userId1 = u.UserId
) AS x
WHERE x.PlayerId = 2; -- Fabiano

Answer 2

Try something like:
SELECT `gamess`.gameId, `users`.userName
FROM users INNER JOIN
(SELECT gameId, userId2 as userId
FROM games
WHERE userId1 = 2
UNION
SELECT gameId, userId1 as userId
FROM games
WHERE userId2 = 2) AS gamess
ON `gamess`.userId = `users`.userId

Answer 3

Doing this without a UNION clause will make it more performant
SELECT g.gameid,
CASE WHEN u1.userid = 2 -- fabino*
THEN u2.username
else u1.username END AS Opponent
FROM games g
LEFT JOIN users u1
ON g.userId1 = u1.UserId
LEFT JOIN users u2
on g.userid2 = u2.userid
WHERE (g.UserId1 = 2 OR g.userid2 = 2) -- fabino

Answer 4

SELECT us.*
FROM users us
INNER JOIN games gs ON us.userId = gs.userId1
GROUP BY us.userId ,us.userName

Answer 5

This query should be works all of common userid
SELECT x.Opponent
FROM
(
SELECT u.userName AS Opponent
FROM games g
INNER JOIN users u
ON g.userId2 = u.UserId
WHERE g.UserId1 in (select UserId from users where UserId in (select userid1 from games))
UNION
SELECT u.userName
FROM games g
INNER JOIN users u
ON g.userId1 = u.UserId
WHERE g.UserId2 in (select UserId from users where UserId in (select userid2 from games))
) AS x;