Hello,
Can I use 2 foreign keys from different tables as a primary key for a third table...?

Leave(leave_type, days,...)

Staff(ID, name,....)

LeaveBalance(LeaveID, balanceDays....)


I need LeaveID to be the primary key of LeaveBalance, where LeaveID comprises of leave_type from Leave and ID from Staff....

Thanks...

You can have a multi-column primary key, if that's what you are asking.

LeaveBalance(Staff_Id, Leave_Type, balanceDays....)

Can't we have composite attributes as primary keys...?

I think that may be specific to the platform you're using. I know you can't in Access or Sql Server.

Of course, every time I use "I know you can't..." somebody else jumps in with
"Sure you can..."

of course, yes you can

What's the difference between having "composite attributes as primary keys" and a standard multi-column (composite) key?

Hold on a second here... If I'm reading this right, "LeaveID comprises of leave_type from Leave and ID from Staff" does that mean that, say, type is a letter and ID is a number so leaveID is going to be X123 or Y456?

You can certainly make that a primary key, but most DBMS's won't let you set up a foreign key relationship that way.

What you *could* do is set up a table LeaveBalanceTbl with two columns, type and ID as the primary key. Then set up a view LeaveBalance that concatenates those columns into one. That would probably work.