Difference between Primary Key, Unique Key and candidate key

Question

What is difference between Primary Key, Unique Key and candidate key?

Answer 1

Super Key is the set of one or more column (ie attributes) which uniquely identifies a record.

Candidate key is a minimal Super key.(it mean we cant remove any attributes from it otherwise it will not remain Superkey anymore).

Primary Key is a arbitrary selected Candidate key. There must be only and only One primary key. We can choose any candidate key as a Primary key. Other candidate keys which are not chosen as Primary are called Alternate Keys.

If Primary Key have more then one column (or attributes) ,it is called Composite Key.