Difference Between Primary key, Unique key And Foreign Key.

The difference between Primary key, Unique key and Foreign Key is the most common interview question for .NET as well as SQL developers.

 The PRIMARY Key and UNIQUE Key constraints, both are similar and enforce uniqueness of the column on which they are defined.

Primary Key

• A primary key cannot have a NULL value.
• Each table can have only one primary key.
• By default, Primary key is clustered index, and the data in the database table is physically organized in the sequence of the clustered index.
• A primary key can be related to other tables as a Foreign Key.
• We can generate ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.
• We can define a Primary key constraint on temporary table and table variable. 
• We can't delete a primary key value from the parent table which is used as a foreign key in the child table. To delete we first need to delete that primary key value from the child table.

Unique Key

• Unique Constraint may have a NULL value.
• Each table can have more than one Unique Constraint.
• By default, Unique key is a unique non-clustered index.
• Unique Constraint cannot be related to another table's as a Foreign Key.

Foreign Key

• A foreign key is a field in the table that is Primary key in another table.
• A foreign key can accept multiple null values.
• A foreign key does not automatically create an index, clustered or non-clustered. You can manually create an index on the foreign key.
• We can have more than one foreign key in a table.
• Foreign keys do not automatically create an index, clustered or non-clustered. You must manually create an index on foreign keys.
• There are actual advantages to having a foreign key be supported with a clustered index, but you get only one per table. What's the advantage? If you are selecting the parent plus all child records, you want the child records next to each other. This is easy to accomplish using a clustered index.
• Having a null foreign key is usually a bad idea instead of NULL  referred to as "orphan record".
• We can’t define a foreign key constraint on a temporary table or table variable.
• We can delete the foreign key value from the child table even though that refers to the primary key of the parent table.