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 ...

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 ...

Occasionally one sees an entity-relationship diagram (ERD) having an unusual characteristic. The unusual element is an entity (or more than one) that seems to have a relationship with every other ...