a sort-of key question

Looking at Darwen and Date's Third Manifesto, chapter four and re-examining the fundamental operations that seem possible within the relational model (to the extent that there is any common agreement about what that is) I wonder how to answer the following question:

What are the last names of the people who share a last name with somebody else?

Suppose the relation R is this:

LAST      FIRST
_____      _____
Smith        Billy
Smith        Bobby
Jones        Nathan

I'd expect the answer to be {Smith}.

Can anybody suggest ways to answer this? (I'd prefer algebraic answers because they are more obviously implemented, but calculus ones might be of some interest.)

thanks for any comments,
pc Received on Sat Mar 19 2005 - 17:07:56 CST