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MySQL UNIQUE
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→ Namedunique_phoneto 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.