Relational database design multiple user types

Question

I have a 4 types of users and each have specific data, but they also share commun data, like username, password ..

My first thought is to create a main users table with user_type column. Then when querying user data i can just first select their user_type and then depending on the output run a different query to grab "user type" specific data. I am not fond of this as i wish i could grab all user related data with one query and ideally using Foreign Keys.

Second idea is to not have a user_type column in the users table and instead use foreign key that from a specific user type table will point to a row the main users table. I like that a bit better though i guess i will have to run N queries, where N is the number of user type every time i need to grab user data.

Are there any other options ? What would be the good practice in such a case ?

Many thanks

Answer 1

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

Answer 2

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)