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

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 relationship the table contains many foreign keys.

But the database does not know whether this is a one-to-one or one-to-many relationship right? The relationships that I make in an ER-Diagram is only to indicate where it should be foreign keys when making the actual tables?

I do not completely grasp the idea of the relationships, even though I have read many tutorials about this.

Thanks in advance.


In a sense, all the relationships we talk about are not known to the database, they are constructs we have invented to better understand how to design the tables.

The big difference in terms of table structure between one-to-one and one-to-many is that in one-to-one it is possible (but not necessary) to have a bidirectional relationship, meaning table A can have a foreign key into table B, and table B can have a foreign key into the associated record in table A. This is not possible with a one-to-many relationship.

One-to-one relationships associate one record in one table with a single record in the other table. One-to-many relationships associate one record in one table with many records in the other table.