Difference between PRIMARY KEY, UNIQUE KEY, CANDIDATE KEY? Where I have to use this?

Question

In my table I have three unique column this can be referred by other table.
Which key I have to use for this.

Answer 1

Primary Key
Primary key is a set of one or more fields/columns of a table that uniquely identify a record in database table. It can not accept null, duplicate values. Only one Candidate Key can be Primary Key.

Candidate Key
A Candidate Key is a set of one or more fields/columns that can identify a record uniquely in a table. There can be multiple Candidate Keys in one table. Each Candidate Key can work as Primary Key.

Unique Key
Uniquekey is a set of one or more fields/columns of a table that uniquely identify a record in database table. It is like Primary key but it can accept only one null value and it can not have duplicate values.