MySQL UNIQUE Constraint: Definition, Syntax, and Examples

The UNIQUE constraint in MySQL ensures that all values in a column or a set of columns are distinct (no duplicates). It is commonly used to prevent duplicate data in database tables.

A PRIMARY KEY constraint automatically has a UNIQUE constraint.

However, you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

UNIQUE Constraint on CREATE TABLE

Example

CREATE TABLE students (

student_id INT PRIMARY KEY,

name VARCHAR(50) NOT NULL,

email VARCHAR(100) UNIQUE,

phone_number VARCHAR(15),

CONSTRAINT unique_phone UNIQUE (phone_number)

);

Explanation:

email → Ensures no two students have the same email.
phone_number → Named unique_phone to ensure phone numbers are unique.

You can apply the UNIQUE constraint to multiple columns, ensuring the combination of values is unique.

Example

CREATE TABLE enrollments (

student_id INT,

course_id INT,

CONSTRAINT unique_enrollment UNIQUE (student_id, course_id)

);

Explanation:

No student can enroll in the same course more than once.

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