How do I create a second primary key/compound key in mysql table

Question

I have a table called enrollments that looks like this:

enrollmentindex - which is table primary key
username - varchar(64)
shortname - varchar(64)
role - varcar(64)

I have to leave the enrollmentindex col, but would like to create a second primary key (compound key) using username, shortname and role.

I am having trouble creating the new compound key because of the existence of the key on enrollmentindex.

Tried:

ALTER TABLE enrollments ADD PRIMARY KEY(username,shortname,role) 

-- results in error code 1068.

ALTER TABLE enrollments DROP PRIMARY KEY, ADD PRIMARY KEY(username,shortname,role)

-- results in error code 1075

There is no data in the table.

Answer 1

Error Code 1068 is for Multiple Primary Keys (not allowed)

Error Code 1075 is for Incorrect Table Definition (auto_increment fields must be defined as a key). It looks like enrollmentindex is an auto_increment field, so it will need to be included in your composite primary key.

I would suggest creating a separate UNIQUE index on username,shortname,role, if that's something you want to force