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

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