UNIQUE constraints in MySQL - Mini Project: Build & Apply

Using UNIQUE Constraints in MySQL

📖 Scenario: You are creating a simple database for a small online bookstore. You want to make sure that each book's ISBN number is unique so that no two books can have the same ISBN.

🎯 Goal: Build a MySQL table called books with columns for id, title, and isbn. Add a UNIQUE constraint on the isbn column to prevent duplicate ISBN numbers.

📋 What You'll Learn

Create a table named books with columns id (integer, primary key, auto-increment), title (varchar 100), and isbn (varchar 20).

Add a UNIQUE constraint on the isbn column.

Use standard MySQL syntax.

💡 Why This Matters

🌍 Real World

Ensuring unique values like ISBNs in a bookstore database prevents data errors and duplicates.

💼 Career

Database developers and administrators often use UNIQUE constraints to maintain data integrity in real applications.

1

Create the books table with columns

Write a MySQL statement to create a table called books with three columns: id as an integer primary key with auto-increment, title as a varchar(100), and isbn as a varchar(20).

MySQL

CREATE TABLE books (
    id INT PRIMARY KEY AUTO_INCREMENT,
    title VARCHAR(100),
    isbn VARCHAR(20)
);
-- Your code here

Hint

Use CREATE TABLE with the column definitions separated by commas. Remember to set id as the primary key and auto-increment.

2

Add a UNIQUE constraint on the isbn column

Modify the books table creation statement to add a UNIQUE constraint on the isbn column to ensure no duplicate ISBNs.

MySQL

CREATE TABLE books (
    id INT AUTO_INCREMENT PRIMARY KEY,
    title VARCHAR(100),
    isbn VARCHAR(20) UNIQUE
);
-- Your code here

Hint

Add the keyword UNIQUE right after the isbn VARCHAR(20) column definition.

3

Insert sample data into the books table

Write two INSERT INTO books statements to add these books:
1. Title: 'Learn SQL', ISBN: '123-4567890123'
2. Title: 'Mastering Databases', ISBN: '987-6543210987'

MySQL

CREATE TABLE books (
    id INT AUTO_INCREMENT PRIMARY KEY,
    title VARCHAR(100),
    isbn VARCHAR(20) UNIQUE
);

INSERT INTO books (title, isbn) VALUES ('Learn SQL', '123-4567890123');
-- Your code here

Hint

Use two separate INSERT INTO books (title, isbn) VALUES (...) statements with the exact titles and ISBNs given.

4

Try inserting a duplicate ISBN to see the UNIQUE constraint in action

Write an INSERT INTO books statement to add a book with title 'Duplicate ISBN Test' and ISBN '123-4567890123' (which already exists). This should fail due to the UNIQUE constraint.

MySQL

CREATE TABLE books (
    id INT AUTO_INCREMENT PRIMARY KEY,
    title VARCHAR(100),
    isbn VARCHAR(20) UNIQUE
);

INSERT INTO books (title, isbn) VALUES ('Learn SQL', '123-4567890123');
INSERT INTO books (title, isbn) VALUES ('Mastering Databases', '987-6543210987');

INSERT INTO books (title, isbn) VALUES ('Duplicate ISBN Test', '123-4567890123');
-- Your code here

Hint

Try inserting a book with the same ISBN as the first book to see the UNIQUE constraint prevent duplicates.

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