Join by null or is there something better?

Question

PostgreSQL

An example to make the problem clear. There is a table:
house number | housing

The problem is that some houses do not have a housing. And I don't know how to be. I don’t like leaving NULL, writing 1 is also not correct, like 0. It could be left like that, but the problem is that I will need to join to another table according to the corpus.

Answer 1

Use a surrogate key to connect.
The number of the house and building cannot be numbers, as they can contain letters. Lack of value is always NULL.