Primary key declaration in MySQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this query?

Given the table users with columns id INT and name VARCHAR(50), and id declared as the primary key, what will this query return?

SELECT COUNT(*) FROM users WHERE id IS NULL;

A Error: Cannot check NULL on primary key

B 1

C NULL

D 0

💡 Hint

Primary keys cannot have NULL values.

Since the primary key column id cannot contain NULL values, the count of rows where id IS NULL is always zero.

📝 Syntax

intermediate

Which option correctly declares a primary key on the order_id column?

You want to create a table orders with a primary key on order_id. Which of the following CREATE TABLE statements is correct?

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

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

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

D CREATE TABLE orders (order_id INT, order_date DATE, PRIMARY order_id);

💡 Hint

The primary key constraint syntax requires parentheses around the column name when declared separately.

Option B correctly uses PRIMARY KEY (order_id) syntax to declare the primary key constraint on the column.

❓ schema

advanced

Which table schema correctly enforces a composite primary key on user_id and session_id?

You want to ensure that the combination of user_id and session_id is unique and not null. Which table definition achieves this?

A CREATE TABLE sessions (user_id INT, session_id INT, PRIMARY KEY (user_id, session_id));

B CREATE TABLE sessions (user_id INT, session_id INT UNIQUE, PRIMARY KEY (user_id));

C CREATE TABLE sessions (user_id INT PRIMARY KEY, session_id INT PRIMARY KEY);

D CREATE TABLE sessions (user_id INT, session_id INT, UNIQUE (user_id, session_id));

💡 Hint

Composite primary keys require listing all columns inside the PRIMARY KEY parentheses.

Option A correctly declares a composite primary key on both user_id and session_id, ensuring uniqueness and non-null for the pair.

🔧 Debug

advanced

Why does this table creation fail?

Consider this SQL statement:

CREATE TABLE products (product_id INT, name VARCHAR(100), PRIMARY KEY product_id);

Why does it cause an error?

A Missing parentheses around product_id in PRIMARY KEY declaration.

B product_id must be declared as NOT NULL explicitly.

C PRIMARY KEY cannot be declared after columns.

D VARCHAR(100) is invalid syntax.

💡 Hint

Check the syntax for PRIMARY KEY constraints.

The PRIMARY KEY declaration requires parentheses around the column name(s). Missing parentheses cause a syntax error.

❓ optimization

expert

Which primary key choice optimizes query speed for a large table with many lookups by email?

You have a large customers table. You want to optimize queries that search by email. Which primary key declaration is best?

A No primary key, just UNIQUE(email)

B PRIMARY KEY (customer_id)

C PRIMARY KEY (email)

D PRIMARY KEY (customer_id, email)

💡 Hint

Primary keys create clustered indexes that speed up lookups.

Declaring email as the primary key creates a clustered index on it, optimizing queries searching by email. Other options either index a different column or do not cluster on email.

Practice

1. What is the main purpose of a PRIMARY KEY in a MySQL table?

easy

A. To uniquely identify each row and prevent duplicates

B. To allow NULL values in a column

C. To speed up SELECT queries only

D. To store large text data

Solution

1. Step 1: Understand the role of PRIMARY KEY

   A PRIMARY KEY uniquely identifies each row in a table and ensures no duplicate or NULL values exist in that column.

2. Step 2: Compare options with PRIMARY KEY purpose

   Only To uniquely identify each row and prevent duplicates correctly states the unique identification and prevention of duplicates, which is the main purpose.

3. Final Answer:

   To uniquely identify each row and prevent duplicates -> Option A

4. Quick Check:

   Primary key = unique row ID [OK]

Hint: Primary key means unique and no duplicates [OK]

Common Mistakes:

• Thinking primary key allows NULL values
• Confusing primary key with indexing only
• Assuming primary key stores large text

2. Which of the following is the correct syntax to declare a primary key inline in a MySQL CREATE TABLE statement?

easy

A. CREATE TABLE users (id INT, name VARCHAR(50 PRIMARY KEY));

B. CREATE TABLE users (id INT, name VARCHAR(50), PRIMARY KEY id);

C. CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50));

D. CREATE TABLE users (id INT, PRIMARY KEY, name VARCHAR(50));

Solution

1. Step 1: Review inline primary key syntax

   Inline primary key declaration is done by adding PRIMARY KEY right after the column type, like id INT PRIMARY KEY.

2. Step 2: Check each option

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); correctly places PRIMARY KEY inline after id INT. Others have syntax errors or misplaced keywords.

3. Final Answer:

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); -> Option C

4. Quick Check:

   Inline primary key = column type + PRIMARY KEY [OK]

Hint: Inline primary key goes right after column type [OK]

Common Mistakes:

• Placing PRIMARY KEY after column list without parentheses
• Using PRIMARY KEY without column name
• Putting PRIMARY KEY after another column

3. Given the table creation below, what will be the result of inserting two rows with the same id?

CREATE TABLE products (
  id INT PRIMARY KEY,
  name VARCHAR(100)
);
INSERT INTO products (id, name) VALUES (1, 'Pen');
INSERT INTO products (id, name) VALUES (1, 'Pencil');

medium

A. Both rows inserted successfully

B. Error due to duplicate primary key value

C. Second row overwrites the first row

D. NULL inserted for the second row's id

Solution

1. Step 1: Understand primary key uniqueness

   The primary key column id must have unique values. Duplicate values cause errors.

2. Step 2: Analyze the insert statements

   The first insert adds id=1. The second insert tries to add the same id=1, violating uniqueness.

3. Final Answer:

   Error due to duplicate primary key value -> Option B

4. Quick Check:

   Duplicate primary key = error [OK]

Hint: Primary key duplicates cause insert errors [OK]

Common Mistakes:

• Assuming duplicate inserts overwrite existing rows
• Thinking NULL can be inserted in primary key
• Believing both rows insert without error

4. Identify the error in the following MySQL table creation statement:

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

medium

A. No error, statement is correct

B. Primary key cannot be declared separately

C. order_id must be VARCHAR type

D. Missing parentheses around the primary key column

Solution

1. Step 1: Check syntax for separate primary key declaration

   When declaring a primary key separately, the column name(s) must be inside parentheses, like PRIMARY KEY (order_id).

2. Step 2: Identify the error in the statement

   The statement uses PRIMARY KEY order_id without parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses around the primary key column -> Option D

4. Quick Check:

   Separate primary key needs parentheses [OK]

Hint: Separate primary key needs ( ) around columns [OK]

Common Mistakes:

• Omitting parentheses in separate primary key
• Thinking primary key must be inline only
• Assuming INT type is invalid for primary key

5. You want to create a table employees with a composite primary key consisting of department_id and employee_number. Which is the correct way to declare this primary key?

hard

A. CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY (department_id, employee_number));

B. CREATE TABLE employees (department_id INT PRIMARY KEY, employee_number INT PRIMARY KEY);

C. CREATE TABLE employees (department_id INT, employee_number INT PRIMARY KEY);

D. CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY department_id, employee_number);

Solution

1. Step 1: Understand composite primary key syntax

   A composite primary key is declared by listing multiple columns inside parentheses after PRIMARY KEY.

2. Step 2: Evaluate each option

   CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY (department_id, employee_number)); correctly declares both columns together as a composite primary key. CREATE TABLE employees (department_id INT PRIMARY KEY, employee_number INT PRIMARY KEY); tries to assign two primary keys separately, which is invalid. CREATE TABLE employees (department_id INT, employee_number INT PRIMARY KEY); declares only one column as primary key. CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY department_id, employee_number); has syntax errors missing parentheses.

3. Final Answer:

   CREATE TABLE employees (department_id INT, employee_number INT, PRIMARY KEY (department_id, employee_number)); -> Option A

4. Quick Check:

   Composite key = PRIMARY KEY (col1, col2) [OK]

Hint: Composite keys list columns inside parentheses [OK]

Common Mistakes:

• Declaring multiple primary keys separately
• Missing parentheses for composite keys
• Assigning primary key to only one column