SQL Server – Difference Between Primary Key and Unique Key In MS Sql Server

Both PRIMARY KEY and UNIQUE KEY enforces the Uniqueness of the values (i.e. avoids duplicate values) on the column[s] on which it is defined. Also these key’s can Uniquely identify each row in database table.

Primary Key:

  • Can be only one in a table
  • It never allows null values
  • Primary Key is a unique key identifier and can not be null
  • By default it adds a clustered index
  • A table can have only one PRIMARY KEY Column[s]
  • We can generated ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.
  • Primary key can be related with another table’s as a Foreign Key.

Unique Key:

  • Can be more than one unique key in one table
  • Unique key can have null values(only single null is allowed)
  • It can be a candidate key
  • Unique key can be null But only one Null value.
  • By default it adds a UNIQUE non-clustered index
  • A table can have more than one UNIQUE Key Column[s]
  • Unique Constraint doesn’t supports Auto Increment value.
  • Unique Constraint can not be related with another table’s as a Foreign Key.

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