Exact difference between Candidate Key,Unique Key,Primary Key with example

Candidate Key is a column or combination of columns, which can be a Primary key for the Table.
There may be multiple Candidate Keys In a Table.

Any Candidate Key can be a Primary Key.

Unique Key is a column which has unique value for each row, but it also allow nulls (Primary Key doesn't).

Any confusions, feel free to ask.

Read - SQL SERVER – Difference Between Candidate Keys and Primary Key[^]

Candidate key is a minimal (i.e. irreducible) key, unique key is a pleonasm, and primary key is an obsoleted concept still surviving from the days when people thought it possible for one key to be "more unique" than any of the others.

A quite excellent information is given regarding your problem here[^].

-KR