table with multiple primary key

[Osmar Alvarez]

mysql> create table test (
-> id int auto_increment,
-> date date,
-> primary key (id,date) );
Query OK, 0 rows affected (0.00 sec)

Ok the above statement works, my question is when I try to do this:

mysql> alter table test add primary key (date);
ERROR 1068 (42000): Multiple primary key defined