Relational database design multiple user types

Your case looks like an instance of class/subclass.

There are two classic ways to design SQL tables to deal with subclasses. Each has advantages and disadvantages.

One way is called "Single Table Inheritance". In this design there is just one table for all types of users. If a given column doesn't pertain to a given row, the intersection is left NULL. A column can be added to indicate the user type.

Another way is called "Class Table Inheritance". This is much like the answer Nanego gave, with a few minor changes. There is one table for users, with all the common data, and a id field. There is one table for each subclass, with data that pertains to that subclass. The id field is often set up as a copy of the id field in the matching row back in the users table. This way the subclass key can do double duty, acting as both a primary key and as a foreign key referencing the user table. This last technique is called "Shared Primary Key". It requires a little programming at insert time, but it's well worth it. It enforces the one-to one nature of the relationship, and it speeds up the necessary joins.

You can look up all three of these designs as tags in SO or as articles out on the web.

single-table-inheritance class-table-inheritance shared-primary-key

Although it's more efficient use of disk space, the problem with splitting into separate tables is that you effectively require conditional joins - joining to the user-type specific table based on the user_type. This is a pain to code as SQL, and these days who cares about disk space?

The better option is to have one user table with enough columns to store information about any user type, knowing that some columns won't be used for some user types. The "inefficiency" of having unused columns will more than be compensated for in execution speed and query simplicity.

It's also easily extendible, in case you get another user type - it's far easier to add columns than it is to add tables, and as you added more user types, the requirement for new columns would diminish (there just aren't that many different things about a user you need to store)

My way to do it would have been to create one table "Person" with the common fields, and one table for each type of user with a foreign key "person_id".

In your request, you just have to join two tables with the foreign_key in order to get all data for one type of user.

How many types of user do you have ?