UNIQUE constraints in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a UNIQUE constraint in a database?

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

beginner

Can a table have multiple UNIQUE constraints?

Yes, a table can have multiple UNIQUE constraints on different columns or sets of columns to ensure uniqueness in each specified area.

intermediate

How does a UNIQUE constraint differ from a PRIMARY KEY?

A PRIMARY KEY uniquely identifies each row and cannot be NULL. A UNIQUE constraint also enforces uniqueness but allows NULL values (unless specified otherwise).

beginner

Write a SQL statement to add a UNIQUE constraint on the 'email' column of a table named 'users'.

ALTER TABLE users ADD CONSTRAINT unique_email UNIQUE (email);

beginner

What happens if you try to insert a duplicate value into a column with a UNIQUE constraint?

The database will reject the insert or update operation and return an error because the UNIQUE constraint is violated.

What does a UNIQUE constraint do in a database table?

A Deletes duplicate rows automatically

B Allows duplicate values but no NULLs

C Automatically creates an index without enforcing uniqueness

D Prevents duplicate values in a column or set of columns

A UNIQUE constraint ensures that all values in the specified column(s) are unique, preventing duplicates.

Can a UNIQUE constraint column contain NULL values?

A No, NULLs are not allowed

B Yes, NULLs are allowed and can appear multiple times

C Yes, but only one NULL is allowed

D It depends on the database system

In MySQL, UNIQUE columns can contain multiple NULL values because NULL is not considered equal to another NULL.

Which SQL command adds a UNIQUE constraint to an existing table?

A ALTER TABLE table_name ADD UNIQUE (column_name);

B CREATE UNIQUE CONSTRAINT table_name (column_name);

C UPDATE TABLE table_name SET UNIQUE = TRUE;

D INSERT UNIQUE INTO table_name (column_name);

The correct syntax to add a UNIQUE constraint is ALTER TABLE with ADD UNIQUE on the column.

How many UNIQUE constraints can a table have?

A Only one

B Two

C Multiple on different columns or column sets

D None

A table can have multiple UNIQUE constraints on different columns or combinations of columns.

What error occurs if you insert a duplicate value into a UNIQUE constrained column?

A Duplicate entry error

B Syntax error

C No error, duplicates allowed

D Connection error

The database returns a duplicate entry error because the UNIQUE constraint is violated.

Explain what a UNIQUE constraint is and how it helps maintain data integrity.

Think about how UNIQUE constraints stop repeated data in a column.

Describe how to add a UNIQUE constraint to an existing table and what happens if a duplicate value is inserted.

Recall the SQL command and the database behavior on 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