I have the following two table scenario:

users
id   groups
1    1,2,3
2    2,3
3    1,3
4    3

and

groups
id
1
2
3

How do I return the IDs of all users that belong to group 2 and 1 for example? Should I look into join, a helper group_membership table or function to separate the comma delimited group IDs to get something like this:

group_membership
user_id   group_id
1         1
1         2
1         3
2         2
2         3
...       ...