You cannot name a PRIMARY KEY. The name PRIMARY KEY is the designation of the preferred unique index (candidate key) for accessing the table. A table can have multiple unique keys.
EXAMPLE: Employee table with three unique keys
- EmployeeID
- Driver's License Number
- Social Security Number
You can create a unique index without it being the PRIMARY KEY as follows:
create table a (foo INTEGER, UNIQUE KEY foo_id (foo));
Check these other links about PRIMARY KEYs vs Unique Keys