UNIQUE constraints in MySQL

Introduction

A UNIQUE constraint makes sure that no two rows in a table have the same value in certain columns. It helps keep data clean and avoids duplicates.

When you want to make sure each email address in a user list is different.

When you want to prevent duplicate product codes in an inventory.

When you want to ensure each username in a system is unique.

When you want to avoid repeated phone numbers in a contact list.

Syntax

MySQL

CREATE TABLE table_name (
  column_name datatype UNIQUE
);

-- Or add UNIQUE constraint after table creation:
ALTER TABLE table_name ADD UNIQUE (column_name);

You can add UNIQUE to one or more columns to prevent duplicate values.

UNIQUE constraints allow NULL values; multiple NULLs are permitted in MySQL.

Examples

Creates a users table where each email must be unique.

MySQL

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

Adds a UNIQUE constraint to the product_code column in the products table.

MySQL

ALTER TABLE products ADD UNIQUE (product_code);

Ensures that the combination of first and last names is unique.

MySQL

CREATE TABLE employees (
  id INT PRIMARY KEY,
  first_name VARCHAR(50),
  last_name VARCHAR(50),
  UNIQUE (first_name, last_name)
);

Sample Program

This creates a customers table with a UNIQUE phone column. The third insert fails because the phone number is already used.

MySQL

CREATE TABLE customers (
  customer_id INT PRIMARY KEY,
  phone VARCHAR(15) UNIQUE
);

INSERT INTO customers (customer_id, phone) VALUES (1, '123-456-7890');
INSERT INTO customers (customer_id, phone) VALUES (2, '987-654-3210');
-- This next insert will fail because phone '123-456-7890' already exists
INSERT INTO customers (customer_id, phone) VALUES (3, '123-456-7890');

Important Notes

UNIQUE constraints help keep your data accurate by stopping duplicates.

If you try to insert a duplicate value, MySQL will give an error and stop the insert.

You can have multiple UNIQUE constraints on different columns in the same table.

Summary

UNIQUE constraints prevent duplicate values in one or more columns.

They help keep data clean and consistent.

Trying to insert duplicates causes an error.

Practice

1. What is the main purpose of a UNIQUE constraint in MySQL?

easy

A. To prevent duplicate values in a column or set of columns

B. To allow NULL values in a column

C. To speed up query performance

D. To automatically generate unique IDs

Solution

1. Step 1: Understand the role of UNIQUE constraint

   A UNIQUE constraint ensures that all values in a column or a group of columns are different from each other.

2. Step 2: Compare with other options

   Options A, C, and D describe other features: allowing NULLs, indexing, and auto-increment, which are not the main purpose of UNIQUE.

3. Final Answer:

   To prevent duplicate values in a column or set of columns -> Option A

4. Quick Check:

   UNIQUE constraint = prevent duplicates [OK]

Hint: UNIQUE means no two rows share the same value [OK]

Common Mistakes:

• Confusing UNIQUE with PRIMARY KEY
• Thinking UNIQUE allows duplicates
• Assuming UNIQUE speeds up queries

2. Which of the following is the correct syntax to add a UNIQUE constraint to an existing column email in a table users?

easy

A. ALTER TABLE users SET UNIQUE email;

B. ALTER TABLE users ADD UNIQUE (email);

C. ALTER TABLE users ADD UNIQUE email;

D. ALTER TABLE users ADD CONSTRAINT UNIQUE email;

Solution

1. Step 1: Recall correct ALTER TABLE syntax for UNIQUE

   The correct syntax is to use ADD UNIQUE (column_name) inside ALTER TABLE.

2. Step 2: Check each option

   ALTER TABLE users ADD UNIQUE (email); matches the correct syntax. Options A, B, and C are invalid or incomplete syntax.

3. Final Answer:

   ALTER TABLE users ADD UNIQUE (email); -> Option B

4. Quick Check:

   ALTER TABLE ADD UNIQUE (column) [OK]

Hint: Use ADD UNIQUE (column) to add constraint [OK]

Common Mistakes:

• Omitting parentheses around column name
• Using SET UNIQUE which is invalid
• Missing ADD keyword

3. Given the table users(id INT, username VARCHAR(50), email VARCHAR(100)) with a UNIQUE constraint on email, what happens if you run this query twice?

INSERT INTO users (id, username, email) VALUES (1, 'alice', '[e-mail]');

medium

A. Both inserts succeed, creating two identical rows

B. Both inserts fail due to UNIQUE constraint

C. Second insert updates the existing row

D. First insert succeeds, second insert fails with duplicate error

Solution

1. Step 1: Understand UNIQUE constraint effect on inserts

   The UNIQUE constraint on email prevents duplicate emails. The first insert adds the row successfully.

2. Step 2: Analyze second insert behavior

   The second insert tries to add the same email again, violating the UNIQUE constraint, causing an error.

3. Final Answer:

   First insert succeeds, second insert fails with duplicate error -> Option D

4. Quick Check:

   UNIQUE blocks duplicate inserts [OK]

Hint: Duplicate insert fails due to UNIQUE constraint [OK]

Common Mistakes:

• Assuming duplicates are allowed
• Thinking second insert updates existing row
• Confusing UNIQUE with PRIMARY KEY behavior

4. You have this table creation statement:

CREATE TABLE products (id INT, code VARCHAR(20) UNIQUE);

You try to insert two rows:

INSERT INTO products VALUES (1, 'X123');
INSERT INTO products VALUES (2, 'X123');

But the second insert fails. What is the most likely cause?

medium

A. The UNIQUE constraint on code prevents duplicate values

B. The id column is missing a PRIMARY KEY

C. The table does not allow NULL values in code

D. The syntax of the INSERT statement is incorrect

Solution

1. Step 1: Identify the UNIQUE constraint effect

   The code column has a UNIQUE constraint, so duplicate values like 'X123' are not allowed.

2. Step 2: Analyze the insert failure reason

   The second insert tries to add 'X123' again, violating the UNIQUE constraint, causing failure.

3. Final Answer:

   The UNIQUE constraint on code prevents duplicate values -> Option A

4. Quick Check:

   UNIQUE constraint blocks duplicate column values [OK]

Hint: UNIQUE blocks duplicate values in that column [OK]

Common Mistakes:

• Thinking PRIMARY KEY is required for uniqueness
• Assuming NULL disallows duplicates
• Blaming syntax error for insert failure

5. You want to ensure that the combination of first_name and last_name in a contacts table is unique, but each column alone can have duplicates. Which is the correct way to enforce this in MySQL?

hard

A. Add a PRIMARY KEY on first_name

B. Add UNIQUE constraints separately on first_name and last_name

C. Add a UNIQUE constraint on (first_name, last_name) together

D. Add a UNIQUE constraint only on last_name

Solution

1. Step 1: Understand composite UNIQUE constraints

   A UNIQUE constraint on multiple columns ensures the combination is unique, but allows duplicates in each column individually.

2. Step 2: Evaluate options for enforcing uniqueness

   Add a UNIQUE constraint on (first_name, last_name) together correctly adds a UNIQUE constraint on the pair (first_name, last_name). Options B, C, and D enforce uniqueness incorrectly or partially.

3. Final Answer:

   Add a UNIQUE constraint on (first_name, last_name) together -> Option C

4. Quick Check:

   Composite UNIQUE = unique pairs, not individual columns [OK]

Hint: Use UNIQUE on multiple columns for combined uniqueness [OK]

Common Mistakes:

• Adding UNIQUE on each column separately
• Confusing PRIMARY KEY with UNIQUE
• Not using composite constraint for pairs