Primary Key vs. Foreign Key vs. Unique Key

Question

I know this is a very basic concept and I predominantly understand the purpose of a Primary Key and a Foreign Key. But as I've been learning the concept of a Unique Key came up and I was unable to find a resource that gives me a clear understanding of the purpose of a Unique Key. I would love it if someone co explain the difference while comparing it to Primary and Foreign Keys potentially with some examples if applicable.

Abhishek Karumuru · Answer

Hi shsingh1 
 Primary Key : A primary key uniquely identifies each record in a table. Each value in the primary key column must be unique. primary key cannot contain NULL values. Every record must have a value for the primary key. The primary key is defined within a single table. 
 Foreign Key : A foreign key is a column or set of columns in one table that refers to the primary key in another table. It establishes a relationship between the two tables. Foreign keys can contain NULL values, depending on the design of the database. The foreign key is defined in a child table and references the primary key of a parent table.