In an application where users can belong to multiple groups, I’m currently storing their groups in a column called groups as a binary. Every four bytes is a 32 bit integer which is the GroupID. However, this means that to enumerate all the users in a group I have to programatically select all users, and manually find out if they contain that group.
Another method was to use a unicode string, where each character is the integer denoting a group, and this makes searching easy, but is a bit of a fudge.
Another method is to create a separate table, linking users to groups. One column called UserID and another called GroupID.
Which of these ways would be the best to do it? Or is there a better way?
You have a many-to-many relationship between users and groups. This calls for a separate table to combine users with groups:
To find all users in a given group, you just say:
Rule of thumb: If you feel like you need to encode multiple values in the same field, you probably need a foreign key to a separate table. Your tricks with byte-blocks or Unicode chars are just clever tricks to encode multiple values in one field. Database design should not use clever tricks – save that for application code 😉