Return to Revisions

How can I set up a foreign key constraint that will know which table to be linked to?

Either you can't, or you shouldn't. :)

The standard way to structure this is to have a parent Entity table, and a table-per-child for each of University, Program, and Course. The Entity table optionally contains a denormalized column that specifies the type of entity -- this has a bunch of advantages, so I recommend it.

Then all you do is create the foreign key to the primary key of Entity.

What I've described looks like this (SQL Server syntax):

CREATE TABLE dbo.EntityType
(
    EntityTypeID tinyint NOT NULL
        PRIMARY KEY
);

CREATE TABLE dbo.Entity
(
    EntityID int NOT NULL IDENTITY
        PRIMARY KEY,
    EntityTypeID tinyint NOT NULL
        FOREIGN KEY REFERENCES dbo.EntityType(EntityTypeID)
);

CREATE TABLE dbo.University
(
    EntityID int NOT NULL
        PRIMARY KEY
        FOREIGN KEY REFERENCES dbo.Entity(EntityID)
);

CREATE TABLE dbo.LearningContext
(
    LearningContextID int NOT NULL IDENTITY
        PRIMARY KEY,
    EntityID int NOT NULL
        FOREIGN KEY REFERENCES dbo.Entity(EntityID)
);

This is the equivalent of subclassing in a database, if you're familiar with client-side programming constructs.

Note how this structure easily allows adding a new derived Entity subtype without changing the existing schema objects.