A set of foreign keys, where all but one are NULL

Question

What is the name of the method for using a set of foreign keys in a table where all but one are NULL for a given row?

In other words, each row requires a foreign key for one (and only one) of n different possible tables, so you have all the necessary foreign keys, but all but one are NULL.

(Django users can recognize this as an alternative to using a universal foreign key)

Answer 1

The description term that you are describing is an Exclusive Arc .

Instead, I prefer to make one foreign key that refers to a common super-table of your n different parent tables.

See my other answers for polymorphic associations:

• Is it possible to use a MySQL foreign key for one of two possible tables?
• Link to foreign keys in the same column
• Why don't you have a foreign key in a polymorphic association?
• fooobar.com/questions/43932 / ...
• How to handle OR relationships in an ERD design (table)?

Answer 2

It would be easier with an example, but a common way to unravel is to simply find a common supertype for these tables. Suppose we have tables Book, Article, Magazine , and now the table should have a foreign key for these tables. Using the common supertype Publication resolves this. See here the model and a similar question / answer.