SQL UNIQUE Constraint - Ensuring Data Uniqueness

What is the SQL UNIQUE Constraint?

The UNIQUE constraint in SQL ensures that all values in a column or a combination of columns are unique across the table.

It prevents duplicate entries, which is essential for maintaining data integrity and avoiding data anomalies.

• Can be applied to one or multiple columns (composite unique constraint).
• Allows NULL values unless restricted by other constraints.
• Multiple UNIQUE constraints can exist in a single table.

How to Use the UNIQUE Constraint

You can define a UNIQUE constraint when creating a table or add it later using ALTER TABLE.

It can be applied to a single column or a set of columns to enforce uniqueness across their combined values.

UNIQUE vs PRIMARY KEY

While both UNIQUE and PRIMARY KEY constraints enforce uniqueness, they have distinct differences.

Understanding these differences helps in designing tables correctly.

• PRIMARY KEY uniquely identifies each row and cannot contain NULL values.
• A table can have only one PRIMARY KEY but multiple UNIQUE constraints.
• UNIQUE constraints allow NULL values (depending on the database system).

Practical Examples

Let's look at some examples demonstrating the UNIQUE constraint in action.