非唯一外键

Question

Do foreign keys have to be unique?

I'm trying to create a table that stores the foreign key that references to a user and a column 'profileIconId'. The purpose of the table is to have a list of icons that the user owns. I would like to use cascade delete.

My other choice is to use SELECT FROM WHERE to retrieve the list and use DELETE FROM WHERE to delete all rows that matches the key when the user is removed.

Answer 1

No, they don't. In fact, one of the most common uses of a foreign key is a one-to-many relationship, such as between Customers and Orders , for example.

Answer 2

No, Foreign Key in a table doesn't have to be unique in said table.

BUT, as it is the Primary Key of another table, it must be unique in this table.

Answer 3

No.

But the values must exists first on the parent table before you can insert it on the table.

Answer 4

No, foreign keys do not have to be unique. Indeed, a lack of uniqueness is requisite for one-to-many or many-to-many relations.