> The way to do it without using a nullable column

I mean, you could, but having separate tables for every optional field would be an organizational and usability nightmare. Queries would be longer and slower for no good reason. Not to mention a gigantic waste of space with all those repeated primary keys and their indexes.

And you could have databases that prohibited NULL values, but we mostly don't, because they're so useful.

> but having separate tables for every optional field would be an organizational and usability nightmare

I think this indicates that declaring and managing state is too onerous in SQL.