Two column as a primary keys in mysql?

Question

Today I found out you can have a primary key using two columns (tsql). The PK must be unique but both columns do not (the combo must be unique).

I thought that was v

Answer 1

Yes you have to be aware of duplicating your PRIMARY_KEY which is a combined key in your case not to be duplicated.

Anyway when you set two PK's this means that your signature is PK1+PK2 so you can duplicate PK1 or PK2 but not the both of them.

Hope that I helped

Answer 2

You are over thinking this:

• primary keys (no matter how many columns are involved) must be unique
• your primary key is the two columns a, b

Therefore a and b together must be unique.

Individual value of a and b are irrelevant.