Difference between Primary Key and Unique Key

The primary key and unique key are both constraints and these are used in relational database management system to make sure data integrity and uniqueness. However, there are several differences present between them: Primary Key

• Primary key used to identify unique records in a table, it contains unique values
• Primary key does not allow null value, it must not have null value
• A table can have only one primary key
• By default primary key creates a indexed on the primary key column

• Unique key used to identify unique records in a table, it contains unique values
• Unique key allow null value, it can have a null value
• A table can have multiple unique key
• By default unique key creates a non-clustered index