Unique indexes in MySQL - Mini Project: Build & Apply

Creating and Using Unique Indexes in MySQL

📖 Scenario: You are managing a small online store database. You want to ensure that each product has a unique product code so that no two products can share the same code.

🎯 Goal: Build a MySQL table for products and add a unique index on the product code to prevent duplicate entries.

📋 What You'll Learn

Create a table named products with columns id, name, and product_code

Add a unique index on the product_code column

Insert sample data with unique product codes

Attempt to insert a duplicate product code to see the unique index in action

💡 Why This Matters

🌍 Real World

Unique indexes are used in databases to prevent duplicate entries for important fields like product codes, email addresses, or user IDs.

💼 Career

Database administrators and backend developers use unique indexes to maintain data integrity and avoid errors caused by duplicate data.

1

Create the products table

Write a SQL statement to create a table called products with three columns: id as an integer primary key, name as a VARCHAR(100), and product_code as a VARCHAR(20).

MySQL

-- Your code here to create the products table

Hint

Use CREATE TABLE with the specified columns and types.

2

Add a unique index on product_code

Write a SQL statement to add a unique index named unique_product_code on the product_code column of the products table.

MySQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(100),
  product_code VARCHAR(20)
);
-- Your code here to add a unique index on product_code

Hint

Use CREATE UNIQUE INDEX followed by the index name and the table and column.

3

Insert sample products with unique codes

Write SQL statements to insert these products into the products table: (1, 'Laptop', 'LP1001') and (2, 'Smartphone', 'SP2002').

MySQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(100),
  product_code VARCHAR(20)
);

CREATE UNIQUE INDEX unique_product_code ON products(product_code);
-- Your code here to insert two products with unique product codes

Hint

Use INSERT INTO with the specified values for each product.

4

Try inserting a duplicate product_code

Write a SQL statement to insert a new product with id 3, name 'Tablet', and product_code 'LP1001' which duplicates an existing product_code to test the unique index.

MySQL

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(100),
  product_code VARCHAR(20)
);

CREATE UNIQUE INDEX unique_product_code ON products(product_code);

INSERT INTO products (id, name, product_code) VALUES (1, 'Laptop', 'LP1001');
INSERT INTO products (id, name, product_code) VALUES (2, 'Smartphone', 'SP2002');
-- Your code here to insert a product with a duplicate product_code 'LP1001'

Hint

Try inserting a product with a product_code that already exists to see the unique index prevent duplicates.

Practice

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

easy

A. To prevent duplicate values in one or more columns

B. To speed up data insertion

C. To allow duplicate values in a column

D. To delete duplicate rows automatically

Solution

1. Step 1: Understand the role of UNIQUE index

   A UNIQUE index ensures that no two rows have the same value in the indexed column(s).

2. Step 2: Compare options with this definition

   Only To prevent duplicate values in one or more columns correctly states that UNIQUE prevents duplicates. Others are incorrect or unrelated.

3. Final Answer:

   To prevent duplicate values in one or more columns -> Option A

4. Quick Check:

   UNIQUE index = prevent duplicates [OK]

Hint: UNIQUE means no duplicates allowed in indexed columns [OK]

Common Mistakes:

• Thinking UNIQUE speeds up inserts
• Believing UNIQUE allows duplicates
• Confusing UNIQUE with automatic deletion

2. Which of the following is the correct syntax to create a UNIQUE index on the column email in a table named users?

easy

A. CREATE UNIQUE KEY idx_email FROM users(email);

B. CREATE INDEX UNIQUE idx_email ON users(email);

C. CREATE UNIQUE INDEX idx_email ON users(email);

D. CREATE UNIQUE ON users INDEX idx_email(email);

Solution

1. Step 1: Recall correct syntax for UNIQUE index creation

   The correct syntax is: CREATE UNIQUE INDEX index_name ON table_name(column_name);

2. Step 2: Match options with correct syntax

   CREATE UNIQUE INDEX idx_email ON users(email); matches exactly. Others have wrong keyword order or syntax errors.

3. Final Answer:

   CREATE UNIQUE INDEX idx_email ON users(email); -> Option C

4. Quick Check:

   CREATE UNIQUE INDEX ... ON ... (column) [OK]

Hint: Remember: CREATE UNIQUE INDEX name ON table(column) [OK]

Common Mistakes:

• Swapping UNIQUE and INDEX keywords
• Using FROM instead of ON
• Incorrect keyword order

3. Given the table employees with a UNIQUE index on employee_id, what happens if you run this query?

INSERT INTO employees (employee_id, name) VALUES (101, 'Alice');
INSERT INTO employees (employee_id, name) VALUES (101, 'Bob');

medium

A. The second insert fails with a duplicate key error

B. Both rows are inserted successfully

C. The second insert overwrites the first row

D. The second insert is ignored silently

Solution

1. Step 1: Understand UNIQUE index effect on inserts

   A UNIQUE index on employee_id prevents duplicate values in that column.

2. Step 2: Analyze the inserts

   The first insert adds employee_id 101. The second insert tries to add the same employee_id 101, violating the UNIQUE constraint, causing an error.

3. Final Answer:

   The second insert fails with a duplicate key error -> Option A

4. Quick Check:

   UNIQUE index blocks duplicate inserts [OK]

Hint: Duplicate insert on UNIQUE column causes error [OK]

Common Mistakes:

• Assuming duplicates are allowed
• Thinking second insert overwrites first
• Believing second insert is ignored silently

4. You tried to create a UNIQUE index on the column username but got an error. Which of these is the most likely cause?

medium

A. The table does not exist

B. The username column contains duplicate values

C. You forgot to specify the index name

D. UNIQUE indexes are not supported on text columns

Solution

1. Step 1: Understand UNIQUE index creation requirements

   Creating a UNIQUE index requires all existing values in the column to be unique.

2. Step 2: Identify common error causes

   If duplicates exist, MySQL throws an error when creating the UNIQUE index.

3. Final Answer:

   The username column contains duplicate values -> Option B

4. Quick Check:

   Duplicates block UNIQUE index creation [OK]

Hint: Check for duplicates before creating UNIQUE index [OK]

Common Mistakes:

• Assuming index name is mandatory
• Believing UNIQUE not allowed on text columns
• Ignoring existing duplicates

5. You want to ensure that the combination of first_name and last_name in the contacts table is unique, but duplicates are still appearing. Which is the best way to fix this?

hard

A. Use a CHECK constraint to prevent duplicates

B. Create separate UNIQUE indexes on first_name and last_name individually

C. Add a PRIMARY KEY on first_name

D. Create a UNIQUE index on both first_name and last_name columns together

Solution

1. Step 1: Understand uniqueness on multiple columns

   To ensure uniqueness on a combination, a composite UNIQUE index on both columns is needed.

2. Step 2: Analyze other options

   Separate UNIQUE indexes allow duplicates across combinations. PRIMARY KEY on one column doesn't cover both. CHECK constraints can't enforce uniqueness.

3. Final Answer:

   Create a UNIQUE index on both first_name and last_name columns together -> Option D

4. Quick Check:

   Composite UNIQUE index = unique pairs [OK]

Hint: Use composite UNIQUE index for multi-column uniqueness [OK]

Common Mistakes:

• Creating separate UNIQUE indexes instead of composite
• Using PRIMARY KEY on one column only
• Relying on CHECK constraints for uniqueness