Difference Between Primary key, Unique key And Foreign Key

Please checkout the below details
Primary Key

  • Primary key cannot have a NULL value.
  • Each table can have only one primary key.
  • By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.
  • Primary key can be related with another table’s as a Foreign Key.
  • We can generated ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.

Foreign Key

  • Foreign key is a field in the table that is primary key in another table.
  • Foreign key can accept multiple null value.
  • Foreign key do not automatically create an index, clustered or non-clustered. You can manually create an index on 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.

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 can not be related with another table’s as a Foreign Key.
  • Unique Constraint doesn’t supports Auto Increment value.

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