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Difference between Primary Key and Unique Key

The difference between primary key and unique key is given bellow:
Primary Key
  1. A table can have only one primary key
  2. A primary key have an implicit NOT NULL constraint
  3. It cannot contain NULL values
Unique Key
  1. A table can have more than one unique key
  2. A unique key have not  implicit NOT NULL constraint
  3. It may or may not contain NULL values
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