Question

A co-worker explained to me that Nullable Foreign Keys are considered bad practice in database design because it violates one of the early normal forms. While I took a basic database class in school, I don’t think that normal forms were discussed as a topic. Wikipedia and other websites that discuss this topic use academic terms, but I’d like to understand this aspect of normal forms better without having to take a whole class on them.

I recognize that not everybody feels nullable foreign keys are bad: this question is not opening up a debate on that matter. Instead, I would like a concrete understanding of how they relate to the topic of Normal Forms. Which Normal Forms are violated by nullable foreign keys, and why?

Answer 1

I think Erwin Smout answers that question in the Q/A you linked.

Date wrote this in the 7th ed. of An Introduction to Database Systems.

First normal form: A relvar is in 1NF if and only if, in every legal
value of that relvar, every tuple contains exactly one value for each
attribute. (p 357)

And also

The whole point about nulls is precisely that they are not
values–they are, to repeat, marks or flags. (p 584)

So in Date’s opinion, a relvar that allows nulls in any column isn’t in 1NF.

His chapter “Missing information” has a lot about this topic.