Question

Let’s say I have a table that represents a super class, students. And then I have N tables that represent subclasses of that object (athletes, musicians, etc). How can I express a constraint such that a student must be modeled in one (not more, not less) subclass?

Clarifications regarding comments:

• This is being maintained manually, not through an ORM package.
• The project this relates to sits atop SQL Server (but it would be nice to see a generic solution)
• This may not have been the best example. There are a couple scenarios we can consider regarding subclassing, and I just happened to invent this student/athlete example.

A) In true object-oriented fashion, it’s possible that the superclass can exist by itself and need not be modeled in any subclasses.

B) In real life, any object or student can have multiple roles.

C) The particular scenario I was trying to illustrate was requiring that every object be implemented in exactly one subclass. Think of the superclass as an abstract implementation, or just commonalities factored out of otherwise disparate object classes/instances.

Thanks to all for your input, especially Bill.

Answer 1

Here are a couple of possibilities. One is a CHECK in each table that the student_id does not appear in any of the other sister subtype tables. This is probably expensive and every time you need a new subtype, you need to modify the constraint in all the existing tables.

CREATE TABLE athletes (   student_id INT NOT NULL PRIMARY KEY,   FOREIGN KEY (student_id) REFERENCES students(student_id),   CHECK (student_id NOT IN (SELECT student_id FROM musicians                        UNION SELECT student_id FROM slackers                        UNION ...))  ); 

edit: @JackPDouglas correctly points out that the above form of CHECK constraint is not supported by Microsoft SQL Server. Nor, in fact, is it valid per the SQL-99 standard to reference another table (see http://kb.askmonty.org/v/constraint_type-check-constraint).

SQL-99 defines a metadata object for multi-table constraints. This is called an ASSERTION, however I don’t know any RDBMS that implements assertions.

Probably a better way is to make the primary key in the students table a compound primary key, the second column denotes a subtype. Then restrict that column in each child table to a single value corresponding to the subtype represented by the table. edit: no need to make the PK a compound key in child tables.

CREATE TABLE athletes (   student_id INT NOT NULL PRIMARY KEY,   student_type CHAR(4) NOT NULL CHECK (student_type = 'ATHL'),   FOREIGN KEY (student_id, student_type) REFERENCES students(student_id, student_type) ); 

Of course student_type could just as easily be an integer, I’m just showing it as a char for illustration purposes.

If you don’t have support for CHECK constraints (e.g. MySQL), then you can do something similar in a trigger.

I read your followup about making sure a row exists in some subclass table for every row in the superclass table. I don’t think there’s a practical way to do this with SQL metadata and constraints. The only option I can suggest to meet this requirement is to use Single-Table Inheritance. Otherwise you need to rely on application code to enforce it.

edit: JackPDouglas also suggests using a design based on Class Table Inheritance. See his example or my examples of the similar technique here or here or here.