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Unique constraint

Unique constraint definition

A unique constraint is a database system rule that ensures every value in a column is unique. Unique constraints play an important role in keeping data consistent and reliable. They’re handy for datasets where data integrity is paramount — for example, ones containing user information such as usernames, IDs, or email addresses.

See also: data entry

How a unique constraint works

  • When creating or modifying a database table, the administrator specifies which column(s) should have a unique constraint. These may be barcodes, reference numbers, employee IDs, or product codes.
  • When a user or system tries to enter data into the table, the database system checks those column(s) to see if the value already exists.
  • If they don’t find the same value in the column, they accept the data for storing.
  • If the value already exists in the column, the database system rejects the data entry, and the user may see an error message.

Unique constraint benefits

  • This rule keeps data consistent and accurate.
  • By enforcing uniqueness, unique constraints make the database more reliable.
  • Unique values can speed up searching for the right information in the dataset. When you know a value is unique, you only need to find it once.
  • Unique constraints make it easy to identify records. This quality is especially useful for columns storing user IDs, email addresses, or product codes.
  • By not allowing duplicate values, they act as an error-checking mechanism, making data entry errors less common.

Unique constraint use cases

  • Primary keys in relational databases
  • Username or email lists
  • Product codes or serial number lists
  • Account numbers in financial systems
  • Barcodes or QR codes in HR systems