Should booleans be NOT NULL in mysql?

Question

In designing my database I realized I left some booleans as NOT NULL and others default to NULL. What is the rule of thumb for when to allow a boolean to be nullable? As the booleans I have are all is_SOMETHING, I'm thinking they should never NULL as they are either classified as the SOMETHING or not.

Answer 1

A NULL boolean has three possible values: true, false, and null. A NOT NULL boolean has only two, true and false. Your application logic will have to suffice for telling you which is the correct choice.