Understanding MySQL Unique Keys

Overview

A Unique Key in MySQL is a constraint that ensures all values in a column are distinct from one another. This means that no two rows can have the same value in that column.

Key Concepts

• Uniqueness: Each value in a Unique Key column must be unique across the table.
• Null Values: Unlike primary keys, a Unique Key can accept null values (but only one null value is allowed for that key).
• Multiple Unique Keys: A table can have multiple Unique Keys.

Benefits of Using Unique Keys

• Data Integrity: Ensures that no duplicate values are entered in designated columns.
• Search Optimization: Improves query performance by creating an index on the unique column.

Example

Creating a Unique Key

You can create a Unique Key when you create a table or alter an existing one.

Creating a Table with a Unique Key:

CREATE TABLE users (
    id INT AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(50) UNIQUE,
    email VARCHAR(100) UNIQUE
);

In this example:

• username and email columns must contain unique values.

Altering a Table to Add a Unique Key

ALTER TABLE users ADD UNIQUE (username);

This command adds a Unique Key to the username column of the existing users table.

Checking for Unique Key Violations

If you attempt to insert a duplicate value into a Unique Key column, MySQL will return an error:

INSERT INTO users (username, email) VALUES ('john_doe', '[email protected]');
INSERT INTO users (username, email) VALUES ('john_doe', '[email protected]'); -- This will fail

Conclusion

Using Unique Keys in MySQL is essential for maintaining data integrity and ensuring that specific fields contain unique data. It allows for efficient data management and search capabilities.