Keys

1. The notions of superkey, candidate key and primary key all apply to the relational model.

2. For example, in Branch-scheme,
   • {bname} is a superkey.
   • {bname, bcity} is a superkey.
   • {bname, bcity} is not a candidate key, as the superkey {bname} is contained in it.
   • {bname} is a candidate key.
   • {bcity} is not a superkey, as branches may be in the same city.
   • We will use {bname} as our primary key.
3. The primary key for Customer-scheme is {cname}.

4. More formally, if we say that a subset of is a superkey for , we are restricting consideration to relations in which no two distinct tuples have the same values on all attributes in . In other words,
   • If and are in , and
   • ,
   • then .