A foreign key is a column or columns of data in one table that refers to the unique data values -- often the primary key data -- in another table. Foreign keys link together two or more tables in a ...

I'm adding a table and subclass table to my DB. (Subclass table = table where the primary key joins to the primary key of another table, to give extra row information to some subset of the rows.

At my company every foreign key has a NOT NULL constraint on it, and if it really needs to be null in some cases, a default value is assigned (like 999999) and given a row in the referring table like ...