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A rule ensuring no two rows have the same value in specified columns.
DESCRIPTION
A unique constraint enforces that a column or column set contains distinct values across all rows. It preserves data integrity, supports keys, and prevents duplicates. Databases implement uniqueness via constraints or unique indexes. Composite uniqueness spans multiple columns to enforce business rules. Proper use improves query performance and consistency while supporting normalization.
ALTERNATIVE DEFINITIONS
PostgreSQL Documentation
A unique constraint ensures that the data contained in a column, or a group of columns, is unique among all the rows in the table.
Explicitly covers single and multi-column uniqueness.
MySQL Reference Manual
A UNIQUE index ensures that all values in the index are distinct.
Describes uniqueness via index mechanism rather than constraint.
Microsoft SQL Server Docs
A UNIQUE constraint enforces the uniqueness of the values in one or more columns.
Highlights enforcement on combinations of columns.
RELATED TERMS
Foreign Key NOT NULL Composite Key Check Constraint Data Integrity Feature Scaling SQL Index
COMPETING TERMS
Primary Key Unique Index
KNOWLEDGE AREAS
Data Storage & Operations Data Engineering Data Modeling Data Management
TARGET AUDIENCE
Data Engineer Data Architect Software Developer Analytics Engineer
EXTERNAL SOURCES
- Wikipedia - Unique key
- PostgreSQL - Constraints overview including UNIQUE
- MySQL - MySQL constraints and unique indexes
- Microsoft - Creating unique indexes in SQL Server
TOP QUESTIONS
What is a Unique Constraint in databases?
It ensures specified columns contain distinct values across rows, preventing duplicates and preserving integrity.
Category: Overview
How does a Unique Constraint compare to a Primary Key?
Primary keys enforce uniqueness and non-nullness as table identifiers; unique constraints allow nulls unless restricted.
Category: Alternatives
When should composite Unique Constraints be used?
Use them when uniqueness spans multiple fields, such as enforcing unique email per tenant or product per catalog.
Category: Example
What are common mistakes with Unique Constraints?
Forgetting multi-column uniqueness, mishandling NULL semantics, and relying on application logic instead of database constraints.
Category: Mistakes
What’s the risk of not enforcing uniqueness?
Duplicates corrupt analytics, break referential integrity, and complicate merges and updates.
Category: Risks
Are Unique Constraints implemented as indexes?
Many databases implement uniqueness via unique indexes under the hood, enabling efficient enforcement and lookups.
Category: Requirements
How do NULLs interact with Unique Constraints?
Behavior varies: some systems allow multiple NULLs; others treat NULL as a value or require NOT NULL.
Category: Misunderstanding
How to add Unique Constraints safely in production?
Backfill data, fix duplicates, create concurrently where supported, and monitor for lock contention.
Category: Management
What’s a hot topic around uniqueness?
Declarative data contracts and schema evolution ensuring uniqueness across microservices and event streams.
Category: Topic