Question

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
...       ...

Answer 1

You should be having a many-to-many relationship between users and groups, (meaning, a user may belong into multiple groups, and a group can hold multiple users).

You do that by having 3 tables:

• users – to describe user information.
• groups – to describe group information.
• user_groups – to describe which users are in which groups.

In the user_groups, you should have only 2 columns, user_id and group_id, each row is a single user belonging in a single group, where repititions on both sides are allowed.

Your example translates into:

user_id   group_id
1         1
1         2
1         3
2         2
2         3
3         1
3         3
4         3

Then, it’s very easy to query all of the users in specific groups, as well as all of the groups as user is in.

This process is also called Database Normalization