Primary Key:- (i) It has unique value and it can’t accept null values.
(ii) We can have only one Primary key in a table.
Unique Key:- (i) It has unique value and it can accept only one null values.
(ii) We can have more than one unique key in a table.
Candidate Key:- candidate key full fill all the requirements of primary key which is not null and have unique records is a candidate for primary key. So thus type of key is known as candidate key. Every table must have at least one candidate key but at the same time can have several.