UNIQUE constraints in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What happens when inserting duplicate values in a UNIQUE column?

Consider a table users with a UNIQUE constraint on the email column. What will be the result of this query?

INSERT INTO users (id, email) VALUES (1, '[e-mail]'), (2, '[e-mail]');

MySQL

CREATE TABLE users (id INT PRIMARY KEY, email VARCHAR(255) UNIQUE);
INSERT INTO users (id, email) VALUES (1, '[e-mail]'), (2, '[e-mail]');

A The second row insertion fails with a duplicate key error.

B The entire insert statement fails and no rows are inserted.

C Both rows insert successfully because the UNIQUE constraint allows duplicates.

D Only the first row inserts; the second row is ignored without error.

💡 Hint

UNIQUE constraints prevent duplicate values in the specified column.

The UNIQUE constraint on the email column prevents duplicate emails. In a multi-row INSERT, if any row violates the constraint, the entire statement fails with a duplicate key error, and no rows are inserted.

🧠 Conceptual

intermediate

How does UNIQUE constraint differ from PRIMARY KEY?

Which statement correctly describes the difference between a UNIQUE constraint and a PRIMARY KEY constraint in MySQL?

A PRIMARY KEY uniquely identifies rows and cannot be NULL; UNIQUE allows multiple NULLs.

B UNIQUE is faster than PRIMARY KEY for indexing.

C PRIMARY KEY allows duplicates; UNIQUE does not.

D UNIQUE constraints automatically create foreign keys.

💡 Hint

Think about NULL values and uniqueness.

PRIMARY KEY columns must be unique and cannot contain NULL values. UNIQUE constraints enforce uniqueness but allow multiple NULLs because NULL is not considered equal to another NULL.

📝 Syntax

advanced

Which statement correctly adds a UNIQUE constraint to an existing column?

You have a table employees with a column phone_number. Which SQL statement correctly adds a UNIQUE constraint to phone_number?

A ALTER TABLE employees MODIFY phone_number UNIQUE;

B ALTER TABLE employees ADD CONSTRAINT UNIQUE phone_number;

C ALTER TABLE employees ADD UNIQUE (phone_number);

D ALTER TABLE employees ADD UNIQUE phone_number;

💡 Hint

Look for the correct syntax to add a UNIQUE constraint with ALTER TABLE.

The correct syntax to add a UNIQUE constraint on a column is: ALTER TABLE table_name ADD UNIQUE (column_name); Options B, C, and D are syntactically incorrect.

❓ optimization

advanced

What is a performance impact of UNIQUE constraints on large tables?

Which statement best describes the performance impact of UNIQUE constraints on large tables during data insertion?

A UNIQUE constraints slow down inserts because the database must check for duplicates.

B UNIQUE constraints speed up inserts by indexing the column.

C UNIQUE constraints have no impact on insert performance.

D UNIQUE constraints cause inserts to fail silently if duplicates exist.

💡 Hint

Think about what the database must do to enforce uniqueness.

When inserting data, the database checks the UNIQUE constraint by searching the index to ensure no duplicates exist. This check adds overhead and slows down inserts compared to columns without constraints.

🔧 Debug

expert

Why does this UNIQUE constraint fail to prevent duplicates?

Given this table definition:

CREATE TABLE orders (order_id INT PRIMARY KEY, customer_id INT, order_date DATE, UNIQUE (customer_id));

Why might duplicates still appear in customer_id after inserting multiple rows?

A Because order_date allows duplicates, which affects customer_id uniqueness.

B Because UNIQUE constraints only apply to PRIMARY KEY columns.

C Because the UNIQUE constraint was not created properly without a name.

D Because NULL values in customer_id are allowed and can appear multiple times.

💡 Hint

Consider how UNIQUE constraints treat NULL values.

UNIQUE constraints allow multiple NULL values in the constrained column because NULL is not considered equal to another NULL. If customer_id is NULL in multiple rows, duplicates appear despite the UNIQUE constraint.

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