3.   Explain the difference between a primary key, a composite key, a super key, a candidate key, and a secondary key.

 

Primary Key

An attribute which uniquely identifies a record (entity instance)

Composite Key

A combination of keys (concatenated key) which can be used to uniquely identify a record (entity instance)

Super Key

Either a Primary key or Composite Key

Candidate Key

A key, or group of keys (composite keys) which may become a primary key

Secondary Key

Any key, or group of keys, which can NOT be considered for entry as a superkey