I made a table named staff_info containing 4 column name,day,duty_start,duty_finish. This table still have no parimary key. Now assume that there are 4 staff and they do their work in each day of the week.That means in each day there work 4 staff and 1 staff work in 7 days. Ultimately i can't either make name or day column as my primary key.But combination of two column value is unique.Is there any way that i can make the combination of these two column as any kind of primary key? I think i made myself clear.