What is the difference between primary key and unique key and foreign

What is the difference between primary key and unique key and foreign key in Oracle?

Answer»

S.no	Primary KEY	Unique Key	FOREIGN Key
1.	Cannot have a NULL value	It can have a NULL value	It can accept the NULL value
2.	It is a clustered index	It is a non-clustered index	The Index is manually created whether clustered or non-clustered
3.	The Primary key can only be 1 in a table	The Unique key can be more than 1 in a table	The Foreign key can be more than 1 in a table