Question: I have a relation that has multiple keys. (For example, in relation Student(SS#,name,address), it could be that (name,address) is one key and SS# is another.) What should I do?
Answer: Multiple keys do occur in practice. In the relational model there is no way to specify multiple keys: you can only underline one set of attributes as the key. If there are other keys that you would like to mention, make a separate note. When you get around to creating actual relations in an actual DBMS, there is a way to specify more than one key.