User Groups

• To add groups you might think to

create table users (
        user_id                 integer primary key,
            ...
        -- a space-separated list of group IDs
        group_memberships       varchar(4000),
        ...
);

• But a much better idea is to create a user_groups table and them create a map:

create table user_group_map (
        user_id         not null references users;
        user_group_id   not null references user_groups;
        unique(user_id, user_group_id)
);