Subqueries in WHERE clause in MySQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all employees whose department ID is in the list returned by the subquery.

MySQL

SELECT * FROM employees WHERE department_id [1] (SELECT id FROM departments WHERE location = 'New York');

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

ANOT

BLIKE

C=

DIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of 'IN' causes an error because '=' expects a single value.

Using 'LIKE' is for pattern matching, not for lists.

Using 'NOT' alone is incomplete and causes syntax errors.

The IN keyword is used to check if a value matches any value in a list returned by a subquery.

2fill in blank

medium

Complete the code to find products with prices greater than the average price returned by the subquery.

MySQL

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

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

A>

B<

C=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' selects products cheaper than average, which is not the goal.

Using '=' selects products exactly at average price, which is too narrow.

Using '<=' includes products priced below or equal to average, not just above.

The query selects products priced higher than the average price, so the comparison operator should be >.

3fill in blank

hard

Fix the error in the query to select customers who have placed orders with total amount greater than 1000.

MySQL

SELECT customer_id FROM orders WHERE total_amount [1] (SELECT SUM(total_amount) FROM orders GROUP BY customer_id);

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

AIN

B=

CANY

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' alone causes an error because the right side is multiple values.

Using '=' expects a single value, causing errors.

Using 'IN' checks for equality, not comparison.

The subquery returns multiple sums (one per customer), so to compare with any of these values, use ANY.

4fill in blank

hard

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

MySQL

SELECT employee_id, salary FROM employees WHERE salary [1] (SELECT AVG(salary) FROM employees WHERE department_id [2] employees.department_id);

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

A>

B=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' or '=' in the first blank selects wrong employees.

Using '!=' in the second blank compares wrong departments.

Mixing up the department_id references causes errors.

The query compares each employee's salary to the average salary in their own department. The first blank uses > to find higher salaries. The second blank uses = to match the department IDs.

5fill in blank

hard

Fill all three blanks to select products whose price is between the minimum and maximum prices in the same category.

MySQL

SELECT product_name, price FROM products WHERE price [1] (SELECT MIN(price) FROM products WHERE category_id [2] products.category_id) AND price [3] (SELECT MAX(price) FROM products WHERE category_id = products.category_id);

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

A>=

B=

C<=

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' or '<' excludes products priced exactly at min or max.

Using '!=' in the second blank causes wrong category matching.

Mixing up comparison operators leads to incorrect results.

The query finds products priced between the minimum and maximum in their category. The first blank uses >= to include prices equal or above the minimum. The second blank uses = to match category IDs. The third blank uses <= to include prices equal or below the maximum.

Practice

1. What does a subquery in the WHERE clause do in MySQL?

easy

A. Updates data in multiple tables at once

B. Filters rows based on the result of another query

C. Creates a new table in the database

D. Deletes rows from a table

Solution

1. Step 1: Understand the role of subqueries in WHERE clause

   A subquery inside a WHERE clause is used to filter rows by comparing values to the result of another query.

2. Step 2: Differentiate from other SQL operations

   Creating tables, updating, or deleting data are not done by subqueries in WHERE; they are separate SQL commands.

3. Final Answer:

   Filters rows based on the result of another query -> Option B

4. Quick Check:

   Subquery in WHERE = filter rows [OK]

Hint: Subqueries in WHERE filter rows using another query's result [OK]

Common Mistakes:

• Thinking subqueries create or modify tables
• Confusing subqueries with JOINs
• Assuming subqueries always return multiple rows

2. Which of the following is the correct syntax for using a subquery in the WHERE clause to find employees with salaries greater than the average salary?

easy

A. SELECT * FROM employees WHERE salary = (SELECT salary FROM employees);

B. SELECT * FROM employees WHERE salary IN SELECT AVG(salary) FROM employees;

C. SELECT * FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);

D. SELECT * FROM employees WHERE salary > ALL SELECT AVG(salary) FROM employees;

Solution

1. Step 1: Check subquery syntax in WHERE clause

   The subquery must be enclosed in parentheses and return a single value for comparison with salary.

2. Step 2: Validate each option's syntax

   Only SELECT * FROM employees WHERE salary > (SELECT AVG(salary) FROM employees); correctly uses parentheses around a single-value subquery. Other options miss parentheses, misuse IN with a single value, use = with a multi-row subquery, or misuse ALL.

3. Final Answer:

   SELECT * FROM employees WHERE salary > (SELECT AVG(salary) FROM employees); -> Option C

4. Quick Check:

   Correct subquery syntax uses parentheses [OK]

Hint: Always enclose subqueries in parentheses in WHERE clause [OK]

Common Mistakes:

• Omitting parentheses around subquery
• Using IN with single-value subquery incorrectly
• Comparing to multiple rows without IN or EXISTS

3. Given the tables orders(order_id, customer_id, amount) and customers(customer_id, city), what will this query return?

SELECT order_id FROM orders WHERE customer_id IN (SELECT customer_id FROM customers WHERE city = 'Paris');

medium

A. All order IDs from customers who live in Paris

B. All order IDs from customers who do not live in Paris

C. All customer IDs from Paris

D. Syntax error due to missing JOIN

Solution

1. Step 1: Understand the subquery result

   The subquery selects customer IDs where city is 'Paris'. This returns a list of customer IDs living in Paris.

2. Step 2: Apply the subquery result in the main query

   The main query selects order IDs where the customer_id is in that list, so orders from Paris customers are returned.

3. Final Answer:

   All order IDs from customers who live in Paris -> Option A

4. Quick Check:

   IN with subquery filters matching customer IDs [OK]

Hint: IN subquery filters rows matching any returned value [OK]

Common Mistakes:

• Confusing orders with customers
• Assuming JOIN is required for this filter
• Thinking it returns customer IDs instead of order IDs

4. Identify the error in this query:

SELECT name FROM employees WHERE department_id = (SELECT department_id FROM departments WHERE location = 'NY' AND active = 1);

Assuming multiple departments can be active in NY.

medium

A. No error, query is correct

B. Missing GROUP BY clause in subquery

C. Incorrect table name 'departments'

D. Subquery returns multiple rows but '=' expects one value

Solution

1. Step 1: Analyze subquery result cardinality

   The subquery may return multiple department_id values because multiple departments can be active in NY.

2. Step 2: Check operator compatibility

   The '=' operator expects a single value, but the subquery returns multiple rows, causing an error.

3. Final Answer:

   Subquery returns multiple rows but '=' expects one value -> Option D

4. Quick Check:

   Use IN for multiple values, '=' for single value [OK]

Hint: Use IN if subquery returns multiple rows, not '=' [OK]

Common Mistakes:

• Using '=' with subquery returning multiple rows
• Assuming subquery always returns one row
• Ignoring error messages about subquery results

5. You want to find all products that have never been ordered. Given tables products(product_id, name) and orders(order_id, product_id), which query correctly uses a subquery in the WHERE clause to achieve this?

hard

A. SELECT name FROM products WHERE product_id NOT IN (SELECT product_id FROM orders);

B. SELECT name FROM products WHERE product_id IN (SELECT product_id FROM orders);

C. SELECT name FROM products WHERE product_id = (SELECT product_id FROM orders);

D. SELECT name FROM products WHERE product_id > (SELECT product_id FROM orders);

Solution

1. Step 1: Understand the goal

   We want products that have no matching product_id in orders, meaning they were never ordered.

2. Step 2: Use NOT IN with subquery

   Using NOT IN with a subquery selecting product_ids from orders filters products not present in orders.

3. Final Answer:

   SELECT name FROM products WHERE product_id NOT IN (SELECT product_id FROM orders); -> Option A

4. Quick Check:

   NOT IN filters out matching values [OK]

Hint: Use NOT IN with subquery to find missing matches [OK]

Common Mistakes:

• Using IN instead of NOT IN for exclusion
• Using '=' with subquery returning multiple rows
• Using comparison operators incorrectly with subqueries