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.