Stuart”) (twitter, “@tomstuart”) (email, “
[email protected]”) (code, “GBP”) (symbol, “£”) (name, “John Doe”) (twitter, “@johndoe”) (email, “
[email protected]”) (code, “GBP”) (symbol, “£”) (name, “Tom Stuart”) (twitter, “@tomstuart”) (email, “
[email protected]”) (code, “USD”) (symbol, “$”) (name, “John Doe”) (twitter, “@johndoe”) (email, “
[email protected]”) (code, “USD”) (symbol, “$”) Rename: ρa/b(R) a and b are attribute names Union: R1 ∪ R2 Difference: R1 - R2 Then there's the product operation, which gives you every combination of records from one relation with records from another. So if you have two users and two currencies, you get four records in the product. And there's also rename, union and difference operations, which I don't have time to go into now.