Question

I have three tables (people, clients_people, clients) that would be joined using

SELECT * FROM people
LEFT JOIN clients_people ON people.id = clients_people.person_id
LEFT JOIN clients ON clients_people.client_id = client.id

Each client record has a location – L1, L2, and L3 (client.location = ‘L1’, etc.). I want the ability to store exclude locations for each person; e.g. I would like to define people.id=1 to exclude clients.location = ‘L1’ and clients.location = ‘L3’ in a new query.

I have two solutions right now.

1. Add column client_location_exclude to the people table and store ‘L1,L3’ in people.id=1 record.
2. Allow for negative integers to be stored in clients_people.client_id that define the exclusions; e.g. clients_people.client_id = -1 defines an L1 exclusion, clients_people.client_id = -2 defines an L2 exclusion, etc.

Are either of these good solutions (pros/cons)? Is there a better (best) solution?

The data is going to be used in a Ruby on Rails application and a PHP (no framework) application; but from my perspective, I’m more interested in how the tables should be structured to hold and easily utilize the exclusion of locations.

Thank you.

Answer 1

The normal form would be to use.. a table (!).. to store the relationship. It is after all a relational database system. If would also make sense if you have a location table, to be able to add referential integrity to the exclusion list. Storing ‘L1,L3’ is bad for not being normalised, and also for being hard to work with down the track (you need string splitting functions to access individual elements all the time).

The table would look like

people_location_exclusions: people_id, location_id

A query could look like this

   SELECT *
     FROM people
LEFT JOIN clients_people
       ON people.id = clients_people.person_id
LEFT JOIN clients
       ON clients_people.client_id = client.id
LEFT JOIN people_location_exclusions
       on people_location_exclusions.people_id = people.id
      and people_location_exclusions.location_id = clients.location_id
    WHERE people_location_exclusions.people_id is NULL

The LEFT JOIN + WHERE clause removes clients from the result even if the client is linked through client-people