unique constraint
compound.compound. a rule in a database that makes sure no two items in a list have the same information. For example, it prevents two different people from having the same email address in a system.
compound. a database rule that ensures all values in a specific column or set of columns are distinct from one another. It prevents duplicate entries while allowing for null values, depending on the specific database engine's implementation.
The system uses a unique constraint to prevent duplicate usernames.
When designing the customer table, the developer added a unique constraint to the tax ID column to ensure no two clients share the same number.
Implementing a unique constraint at the database level provides a final layer of data integrity that prevents race conditions from creating duplicate records during high-traffic periods.
A compositional term formed from the Latin unicus, meaning single or sole, and the Old French constrainte, derived from constringere, to bind together.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.
Commonly used in the context of SQL and relational database design; often applied to columns that serve as natural keys.
Last updated July 22, 2026