# unique constraint

CEFR C1 · Compound

Source: https://englishreference.com/word/unique-constraint
Updated: 2026-07-22

## Definition

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.

For teachers: 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.

## Examples

- 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.

## Usage notes

- Commonly used in the context of SQL and relational database design; often applied to columns that serve as natural keys.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.