How to query two same values in SQL Server 2008? - sql

Question

I have table in SQL Server 2008 with following fields
RoomUserId->Primary key
RoomId->Foreign Key of Table Rooms
UserId->Foreign Key of Table Users
Now I have the values as following where the RoomId is common for both users
RoomUserId RoomId UserId
1 11 1
2 11 2
3 12 1
4 12 3
5 13 1
6 13 4
Now I need a SQL query to find the roomid of two users which is distinct. i.e, roomid of user 1 and user 2, roomid of user 1 and user 3.
Please anyone help me with is since I am new to SQL Server.

Answer 1

Try this:
SELECT r1.roomId FROM room r1 JOIN room r2 ON r1.roomId = r2.roomId WHERE r1.userId = 1 AND r2.userId = 3

Answer 2

If you mean find a roomid that is common to users:
select distinct f.roomid
from rooms f
inner join rooms s on f.roomid = s.roomid and f.userid <> s.userid
Or you can use grouping:
select roomid
from rooms
group by roomid
having count(distinct userid) > 1

Answer 3

If you only ever need rooms where there is more than one user then this will work:
SELECT DISTINCT RoomID
FROM RoomUser r1
INNER JOIN RoomUser r2
ON r1.RoomID = r2.RoomID
AND r1.RoomUserID != r2.RoomUserID
If you need the room ID of rooms with x users then use the Having Clause, this is more extensible than self joining e.g. if you need to find room IDs with 3 or more User IDs then you would end up with:
SELECT RoomID
FROM RoomUser
GROUP BY RoomID
HAVING COUNT(DISTINCT UserID) > 3
Whereas using self joins, while probably more efficient will end up with some quite messy SQL. Check execution plans and run some tests to see which is more efficient for your needs.
If you actually need the user IDs of the Users in Rooms with more than one User ID then you could use a CTE to build a comma separated string of users in each room:
;WITH RoomUserCTE AS
( SELECT RoomID,
MIN(UserID) [UserID],
CONVERT(VARCHAR(20), MIN(UserID)) [Users],
0 [Recursion]
FROM RoomUser
GROUP BY RoomID
UNION ALL
SELECT a.RoomID,
b.UserID [UserID],
CONVERT(VARCHAR(20), Users + ', ' + CONVERT(VARCHAR, b.UserID)),
Recursion + 1
FROM RoomUserCTE a
INNER JOIN RoomUser b
ON a.RoomID = b.RoomID
AND b.UserID > a.UserID
)
SELECT RoomID, Users
FROM ( SELECT *, MAX(Recursion) OVER(PARTITION BY RoomID) [MaxRecursion]
FROM RoomUserCTE
) cte
WHERE MaxRecursion = Recursion
For the data in your question this will yield
| RoomID | Users |
|---------+---------|
| 11 | 1, 2 |
| 12 | 1, 3 |
| 13 | 1, 4 |
This would work no matter how many user IDs were associated with the same Room ID, so again is more forward compatible.