Difference between one-to-one and one-to-many relationship in database

Question

This is probably a basic(dumb) question but when having a one-to-one relationship in a database the other table has a foreign key ID(in this example). And in a one-to-many relat

Answer 1

Given table A and B if

1. A and B have a strict 1 to 1 relationship
2. For every B instance, there will always be an A instance

The best approach is to make the primary key of B also a foreign key referencing A. This is also called "Table per Type Inheritance" and the "is a" relationship. There are other ways to enforce a unique foreign key, but using the primary key makes the relationship clear in the schema and in ER diagrams.

Of course there are always other scenarios, and if your design doesn't meet both of the criteria above, you'll have to use another approach.