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

To make it easier to explain, I will only use one table. Create a table with 2 int columns, and a PK on both of them together. As in the question.

create table test(
  a  INT  NOT NULL ,
  b  INT  NOT NULL ,
  PRIMARY KEY(a,b));

Now we can add rows, until we get an error

insert into test values(1,1); 
Query OK, 1 row affected (0,00 sec)

insert into test values(1,2); 
Query OK, 1 row affected (0,00 sec)

insert into test values(1,1); 
ERROR 1062 (23000): Duplicate entry '1-1' for key 'PRIMARY'

Which is logical because the combined values of the 2 columns which make up the PK are not unique anymore when this last statement would be executed.

It is allowed to store 2x the value 1 in a, because that is not the PK. The PK is the combined value of columns a and b.