Keys ( Database )

* Let K Í R

* K is a superkey of R if values for K are sufficient to identify a unique tuple of each possible relation r(R)

# by “possible r ” we mean a relation r that could exist in the enterprise we are modeling.

# Example:  {customer_name, customer_street} and
                 {customer_name}
are both superkeys of Customer, if no two customers can possibly have the same name

4In real life, an attribute such as customer_id would be used instead of customer_name to uniquely identify customers, but we omit it to keep our examples small, and instead assume customer names are unique.

Keys (Cont.)

nK is a candidate key if K is minimal
Example:  {customer_name} is a candidate key for Customer, since it is a superkey and no subset of it is a superkey.

nPrimary key: a candidate key chosen as the principal means of identifying tuples within a relation

lShould choose an attribute whose value never, or very rarely, changes.

lE.g. email address is unique, but may change