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MySQL UNIQUE

~1 min read · MYSQL
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MySQL UNIQUE Constraint

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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