The problem that the OP wants to solve is just one instance of a very general problem: doing "set arithmetic" on two lists: intersection, union, exclusive-or.

I tend to encounter problems of this type so often in my work that I wrote a very general tool to address it -- you can find it here: cmpcol.

It allows that one or both input lists may actually be flat tables (possibly with varied field delimiters), where just one or more columns are of interest for doing the set-arithmetic.