Question

I’m sure there is a proper word for this which I fail to remember, but the problem is easy to describe:
I have a table groupmembers, which is a simple relationship between groups and members:

id | groupid | memberid
1  | g1      | m1
2  | g1      | m2
3  | g2      | m1
4  | g2      | m2
5  | g2      | m3

Above describing two groups, one with m1 and m2 and one with m1,m2 and m3.
If I want to select groupids which has members m1,m2 but no other members, how do I do it? The approaches I have tried would also return g2, as m1 and m2 is a subset of them.

UPDATE: Wow, some great answers! Let me first clarify my question a little – I want to be able to select the group that exactly matches the given members m1 and m2. So, it should NOT match if the group also contains more members than m1 and m2, and it should NOT match if the group contains less than members m1 and m2.

Answer 1

from your phrase

I want to select groupids which has members m1,m2 but no other members

try this one, the idea behind is to count the total instances of records that match the condition and the where clause and that it is equal to the total number of records per group.

SELECT groupid
FROM table1 a
WHERE memberid IN ('m1','m2')
GROUP BY groupid
HAVING COUNT(*) = 
(
  SELECT COUNT(*)
  FROM table1 b
  WHERE b.groupid = a.groupid
  GROUP BY b.groupID
)

SQLFiddle Demo