Maintaining subclass integrity in a relational database

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.

Answers


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.


Each Student record will have a SubClass column (assume for the sake of argument it's a CHAR(1)). {A = Athlete, M=musician...}

Now create your Athlete and Musician tables. They should also have a SubClass column, but there should be a check constraint hard-coding the value for the type of table they represent. For example, you should put a default of 'A' and a CHECK constraint of 'A' for the SubClass column on the Athlete table.

Link your Musician and Athlete tables to the Student table using a COMPOSITE foreign key of StudentID AND Subclass. And you're done! Go enjoy a nice cup of coffee.

CREATE TABLE Student (
    StudentID INT NOT NULL IDENTITY PRIMARY KEY,
    SubClass CHAR(1) NOT NULL,
    Name VARCHAR(200) NOT NULL,
    CONSTRAINT UQ_Student UNIQUE (StudentID, SubClass)
);

CREATE TABLE Athlete (
    StudentID INT NOT NULL PRIMARY KEY,
    SubClass CHAR(1) NOT NULL,
    Sport VARCHAR(200) NOT NULL,
    CONSTRAINT CHK_Jock CHECK (SubClass = 'A'),
    CONSTRAINT FK_Student_Athlete FOREIGN KEY (StudentID, Subclass) REFERENCES Student(StudentID, Subclass)
);

CREATE TABLE Musician (
    StudentID INT NOT NULL PRIMARY KEY,
    SubClass CHAR(1) NOT NULL,
    Instrument VARCHAR(200) NOT NULL,
    CONSTRAINT CHK_Band_Nerd CHECK (SubClass = 'M'),
    CONSTRAINT FK_Student_Musician FOREIGN KEY (StudentID, Subclass) REFERENCES Student(StudentID, Subclass)
);

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