Some database management tools refuse to admit you can add multiple primary key columns to one table, so you might have to do it with direct sql syntax ( deep intake of breath ).
Multiple primary keys make ‘ON DUPLICATE KEY UPDATE column = column + 100’ type queries a shed load more flexible.
Anyway, on with the code:
ALTER TABLE tablename ADD PRIMARY KEY (column_one, column_two)
And there you have it. Robert’s your fathers brother.