Why subqueries are needed in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select employees who earn more than the average salary.

SQL

SELECT name FROM employees WHERE salary > (SELECT [1] FROM employees);

Drag options to blanks, or click blank then click option'

AMIN(salary)

BMAX(salary)

CAVG(salary)

DCOUNT(salary)

Attempts:

3 left

💡 Hint

Common Mistakes

Using MAX() instead of AVG() in the subquery.

Forgetting to use a subquery for comparison.

The subquery calculates the average salary. The main query selects employees earning more than this average.

2fill in blank

medium

Complete the code to find products with prices higher than the average price.

SQL

SELECT product_name FROM products WHERE price > (SELECT [1] FROM products);

Drag options to blanks, or click blank then click option'

AAVG(price)

BSUM(price)

CMIN(price)

DCOUNT(price)

Attempts:

3 left

💡 Hint

Common Mistakes

Using SUM() instead of AVG() in the subquery.

Not using parentheses for the subquery.

The subquery calculates the average price. The main query selects products priced above this average.

3fill in blank

hard

Fix the error in the query to select customers who placed orders with amounts greater than the average order amount.

SQL

SELECT customer_id FROM orders WHERE order_amount > (SELECT [1](order_amount) FROM orders);

Drag options to blanks, or click blank then click option'

AMAX

BAVG

CSUM

DMIN

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the aggregate function.

Using SUM or MAX instead of AVG.

The subquery needs the AVG function to calculate the average order amount. Without AVG, the query is invalid.

4fill in blank

hard

Fill both blanks to select employees whose salary is greater than the average salary in their department.

SQL

SELECT name FROM employees WHERE salary > (SELECT [1] FROM employees WHERE department_id = [2]);

Drag options to blanks, or click blank then click option'

AAVG(salary)

BMAX(salary)

Cdepartment_id

DMIN(salary)

Attempts:

3 left

💡 Hint

Common Mistakes

Using MAX instead of AVG for salary.

Using wrong column for department filter.

The subquery calculates the average salary for the employee's department. The main query compares each employee's salary to this average.

5fill in blank

hard

Fill all three blanks to select products with price greater than the average price in their category and category matches the main query.

SQL

SELECT product_name FROM products WHERE price > (SELECT [1] FROM products WHERE category_id = [2]) AND category_id = [3];

Drag options to blanks, or click blank then click option'

AAVG(price)

Bcategory_id

DMAX(price)

Attempts:

3 left

💡 Hint

Common Mistakes

Using MAX(price) instead of AVG(price).

Not matching category_id in both places.

The subquery calculates the average price for the category. The main query selects products priced above this average and in the same category.

Practice

1. Why do we use subqueries in SQL?
SELECT * FROM employees WHERE department_id IN (SELECT id FROM departments WHERE location = 'NY');

easy

A. To use the result of one query inside another for filtering or comparison

B. To speed up the database server automatically

C. To create new tables from existing ones

D. To change the database schema

Solution

1. Step 1: Understand the role of subqueries

   The subquery inside the IN clause fetches department IDs located in 'NY'.

2. Step 2: See how the main query uses subquery results

   The main query selects employees whose department_id matches those IDs from the subquery.

3. Final Answer:

   To use the result of one query inside another for filtering or comparison -> Option A

4. Quick Check:

   Subqueries help filter data using other query results [OK]

Hint: Subqueries let queries talk to each other inside SQL [OK]

Common Mistakes:

• Thinking subqueries speed up queries automatically
• Confusing subqueries with table creation
• Believing subqueries change database structure

2. Which of the following is the correct syntax for a subquery in SQL?

easy

A. SELECT name FROM employees WHERE id IN SELECT manager_id FROM departments WHERE id = 5;

B. SELECT name FROM employees WHERE id == (SELECT manager_id FROM departments WHERE id = 5);

C. SELECT name FROM employees WHERE id = SELECT manager_id FROM departments WHERE id = 5;

D. SELECT name FROM employees WHERE id = (SELECT manager_id FROM departments WHERE id = 5);

Solution

1. Step 1: Check correct subquery syntax

   Subqueries must be enclosed in parentheses and use a single equals sign for comparison.

2. Step 2: Identify syntax errors in other options

   SELECT name FROM employees WHERE id == (SELECT manager_id FROM departments WHERE id = 5); uses '==' which is invalid in SQL; C misses parentheses; D misses parentheses around subquery.

3. Final Answer:

   SELECT name FROM employees WHERE id = (SELECT manager_id FROM departments WHERE id = 5); -> Option D

4. Quick Check:

   Subqueries need parentheses and single '=' [OK]

Hint: Subqueries always go inside parentheses with '=' or IN [OK]

Common Mistakes:

• Using '==' instead of '=' for comparison
• Forgetting parentheses around subqueries
• Using subqueries without proper syntax

3. What will be the output of this query?

SELECT name FROM employees WHERE department_id = (SELECT id FROM departments WHERE name = 'Sales');

Assuming the departments table has one row with name 'Sales' and id 3, and employees table has:
id | name | department_id
1 | Alice | 3
2 | Bob | 2
3 | Carol | 3

medium

A. No rows returned

B. Bob only

C. Alice and Carol

D. Alice, Bob, and Carol

Solution

1. Step 1: Find department id for 'Sales'

   The subquery returns id = 3 for 'Sales' department.

2. Step 2: Select employees with department_id = 3

   Employees Alice and Carol have department_id 3, so they are selected.

3. Final Answer:

   Alice and Carol -> Option C

4. Quick Check:

   Subquery returns 3, employees with department_id 3 selected [OK]

Hint: Match subquery result with main query filter [OK]

Common Mistakes:

• Assuming subquery returns multiple rows causing error
• Selecting employees from wrong department
• Ignoring subquery result in main query

4. Identify the error in this SQL query:

SELECT name FROM employees WHERE department_id = SELECT id FROM departments WHERE location = 'LA';

medium

A. Missing parentheses around the subquery

B. Using '=' instead of 'IN' for multiple results

C. Wrong table name 'employees'

D. No error, query is correct

Solution

1. Step 1: Check subquery syntax

   The subquery must be enclosed in parentheses to be valid.

2. Step 2: Confirm other parts

   Using '=' is okay if subquery returns one value; table names are correct.

3. Final Answer:

   Missing parentheses around the subquery -> Option A

4. Quick Check:

   Subqueries need parentheses [OK]

Hint: Always put subqueries inside parentheses [OK]

Common Mistakes:

• Forgetting parentheses around subqueries
• Assuming '=' works for multiple rows
• Misreading table names as errors

5. You want to find all customers who placed orders with a total amount greater than the average order amount. Which query correctly uses a subquery to achieve this?

hard

A. SELECT customer_id FROM orders WHERE total_amount > AVG(total_amount);

B. SELECT customer_id FROM orders WHERE total_amount > (SELECT AVG(total_amount) FROM orders);

C. SELECT customer_id FROM orders WHERE total_amount IN (SELECT AVG(total_amount) FROM orders);

D. SELECT customer_id FROM orders WHERE total_amount = (SELECT total_amount FROM orders WHERE total_amount > AVG(total_amount));

Solution

1. Step 1: Understand the goal

   We want customers with orders greater than the average order amount.

2. Step 2: Check subquery usage

   SELECT customer_id FROM orders WHERE total_amount > (SELECT AVG(total_amount) FROM orders); correctly uses a subquery to calculate average and compares each order's total_amount to it.

3. Step 3: Identify errors in other options

   SELECT customer_id FROM orders WHERE total_amount > AVG(total_amount); misuses AVG without subquery; C uses IN incorrectly; D has wrong comparison logic.

4. Final Answer:

   SELECT customer_id FROM orders WHERE total_amount > (SELECT AVG(total_amount) FROM orders); -> Option B

5. Quick Check:

   Subquery calculates average, main query compares amounts [OK]

Hint: Use subquery to get average, compare in main query [OK]

Common Mistakes:

• Using aggregate functions without subqueries
• Misusing IN for single value comparisons
• Comparing with wrong operators or missing parentheses