What is a primary key, foreign key, unique key ?

 1) What is a primary key?

A primary key is a field or the combination of fields which uniquely specify a row. The Primary key is a special kind of unique key. Primary key values cannot be NULL. For example, the Social Security Number can be treated as the primary key for any individual.

2) What is a foreign key?

A foreign key is specified as a key which is related to the primary key of another table. A relationship needs to be created between two tables by referencing foreign key with the primary key of another table. Foreign key acts like a cross-reference between tables as it refers to the primary key of other table and the primary key-foreign key relationship is a very crucial relationship as it maintains the ACID properties of database sometimes.

3) What is a unique key?

Unique key constraint uniquely identifies each record in the database. This key provides uniqueness for the column or set of columns.

The Unique key cannot accept a duplicate value.

The unique key can accept only on Null value.

4) What is the difference between primary key and unique key?

Primary key and unique key both are the essential constraints of the SQL, but there is a small difference between them

Primary key carries unique value but the field of the primary key cannot be Null on the other hand unique key also carry unique value but it can have a single Null value field.