Difference between Primary Key, Unique Key and candidate key

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.

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.

Candidate keys - It is a column that can uniquely identify all columns for a specific row or instance of your database table. Your passport ID is a good example of a Candidate key and when using this ID you can retrieve a persons name, surname and other details related to that specific person.

Primary key - There may be a few Candidate keys on a specific database table which is unique and can be used to identify a specific instance in a table e.g. Vehicle registration number, Chassis number, engine serial number etc... but remember there can only be one candidate key that will be used as a primary key. Difference between Candidate Key vs Primary Key:-) Both Primary and Candidate keys can uniquely identify records in a table on database.

2) Both Primary and Candidate keys are has constraints UNIQUE and NOT NULL.

3) Primary key or Candidate keys can be either single column or combination of multiple columns in a table.