What is candidate Key

candidate Key [Primary Key] is a key which maintains the Row Unique.
it Can be defined based on the entity. composite Key can be either Primay or Unique Key.
More then One Key columns are said to be composite keys. Candidate Key(Primary Key) is a column in a table which has the ability to become a primary key.But a primary key does not necessarily have to be the target of any foreign keys. The candidate key must be unique within its domain.