Subqueries with IN operator in MySQL

Introduction

Subqueries with the IN operator help you find rows where a value matches any value from a list returned by another query.

When you want to find all customers who have placed orders.

When you want to list products that appear in any sales records.

When you want to filter employees who work in departments located in a specific city.

When you want to get students enrolled in any of the courses offered this semester.

Syntax

MySQL

SELECT column_name(s)
FROM table_name
WHERE column_name IN (SELECT column_name FROM another_table WHERE condition);

The subquery inside the IN operator returns a list of values to match against.

The main query returns rows where the column value is found in that list.

Examples

Finds customers who have made at least one order.

MySQL

SELECT name
FROM customers
WHERE id IN (SELECT customer_id FROM orders);

Lists products in the 'Electronics' category.

MySQL

SELECT product_name
FROM products
WHERE category_id IN (SELECT id FROM categories WHERE name = 'Electronics');

Finds employees working in departments located in New York.

MySQL

SELECT employee_name
FROM employees
WHERE department_id IN (SELECT id FROM departments WHERE location = 'New York');

Sample Program

This query finds employees who work in departments located in New York.

MySQL

CREATE TABLE departments (id INT, name VARCHAR(20), location VARCHAR(20));
INSERT INTO departments VALUES (1, 'Sales', 'New York'), (2, 'HR', 'Chicago'), (3, 'IT', 'New York');

CREATE TABLE employees (id INT, employee_name VARCHAR(20), department_id INT);
INSERT INTO employees VALUES (1, 'Alice', 1), (2, 'Bob', 2), (3, 'Charlie', 3), (4, 'Diana', 2);

SELECT employee_name
FROM employees
WHERE department_id IN (SELECT id FROM departments WHERE location = 'New York');

Important Notes

Subqueries with IN are useful for filtering based on related data.

Make sure the subquery returns only one column.

Using IN with a subquery can be slower on large data; indexes help improve speed.

Summary

Use IN with subqueries to match values from another table.

Subqueries return a list of values for the main query to check.

This helps connect related data easily without complex joins.

Practice

1. What does the IN operator do when used with a subquery in MySQL?

easy

A. Checks if a value matches any value returned by the subquery

B. Joins two tables based on a common column

C. Updates values in the main table using subquery results

D. Deletes rows that match the subquery results

Solution

1. Step 1: Understand the IN operator

   The IN operator checks if a value exists within a list of values.

2. Step 2: Role of subquery with IN

   The subquery returns a list of values, and IN checks if the main query's value matches any of these.

3. Final Answer:

   Checks if a value matches any value returned by the subquery -> Option A

4. Quick Check:

   IN with subquery = value match list [OK]

Hint: IN checks if value is in subquery result list [OK]

Common Mistakes:

• Confusing IN with JOIN
• Thinking IN updates or deletes rows
• Assuming IN returns a single value

2. Which of the following is the correct syntax to use a subquery with the IN operator in MySQL?

easy

A. SELECT * FROM employees WHERE department_id = IN (SELECT id FROM departments);

B. SELECT * FROM employees WHERE department_id IN (SELECT id FROM departments);

C. SELECT * FROM employees WHERE IN department_id (SELECT id FROM departments);

D. SELECT * FROM employees WHERE department_id IN SELECT id FROM departments;

Solution

1. Step 1: Review correct IN syntax

   The IN operator must be followed by parentheses enclosing the subquery.

2. Step 2: Check each option

   SELECT * FROM employees WHERE department_id IN (SELECT id FROM departments); correctly uses IN with parentheses and a subquery. Others have syntax errors.

3. Final Answer:

   SELECT * FROM employees WHERE department_id IN (SELECT id FROM departments); -> Option B

4. Quick Check:

   IN + (subquery) = correct syntax [OK]

Hint: Use IN followed by parentheses around subquery [OK]

Common Mistakes:

• Adding = before IN
• Missing parentheses around subquery
• Placing IN before column name

3. Given the tables:
employees(id, name, department_id)
departments(id, name)
What will this query return?

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

medium

A. An error because subquery returns multiple rows

B. Names of all employees regardless of department

C. Names of employees who work in the Sales department

D. Names of employees who do not work in Sales

Solution

1. Step 1: Analyze subquery

   The subquery selects department IDs where the department name is 'Sales'.

2. Step 2: Main query filters employees

   The main query selects employee names whose department_id matches any ID from the subquery.

3. Final Answer:

   Names of employees who work in the Sales department -> Option C

4. Quick Check:

   IN filters employees by Sales department IDs [OK]

Hint: Subquery filters IDs; IN matches employees with those IDs [OK]

Common Mistakes:

• Thinking IN causes error with multiple rows
• Ignoring subquery filter condition
• Assuming it returns all employees

4. Identify the error in this query:

SELECT * FROM orders WHERE customer_id IN SELECT id FROM customers WHERE city = 'New York';

medium

A. Subquery returns no rows

B. Incorrect table name 'orders'

C. Using IN instead of EXISTS

D. Missing parentheses around the subquery after IN

Solution

1. Step 1: Check IN operator syntax

   The IN operator requires the subquery to be enclosed in parentheses.

2. Step 2: Identify missing parentheses

   The query lacks parentheses around the subquery after IN, causing syntax error.

3. Final Answer:

   Missing parentheses around the subquery after IN -> Option D

4. Quick Check:

   IN needs (subquery) [OK]

Hint: Always put subquery inside parentheses after IN [OK]

Common Mistakes:

• Omitting parentheses around subquery
• Confusing IN with EXISTS
• Assuming table names cause error

5. You want to find all products that have been ordered by customers from the city 'Chicago'.
Given tables:
products(id, name)
orders(id, product_id, customer_id)
customers(id, city)
Which query correctly uses a subquery with IN to get the product names?

hard

A. SELECT name FROM products WHERE id IN (SELECT product_id FROM orders WHERE customer_id IN (SELECT id FROM customers WHERE city = 'Chicago'));

B. SELECT name FROM products WHERE id = IN (SELECT product_id FROM orders WHERE customer_id = IN (SELECT id FROM customers WHERE city = 'Chicago'));

C. SELECT name FROM products WHERE id IN SELECT product_id FROM orders WHERE customer_id IN SELECT id FROM customers WHERE city = 'Chicago';

D. SELECT name FROM products WHERE id IN (SELECT id FROM customers WHERE city = 'Chicago');

Solution

1. Step 1: Understand the relationships

   Products link to orders by product_id; orders link to customers by customer_id.

2. Step 2: Build nested subqueries

   Inner subquery gets customer IDs from Chicago; middle subquery gets product_ids from orders by those customers; outer query selects product names with those IDs.

3. Final Answer:

   SELECT name FROM products WHERE id IN (SELECT product_id FROM orders WHERE customer_id IN (SELECT id FROM customers WHERE city = 'Chicago')); -> Option A

4. Quick Check:

   Nested IN subqueries correctly filter products [OK]

Hint: Use nested IN subqueries to filter related tables stepwise [OK]

Common Mistakes:

• Adding = before IN
• Missing parentheses around subqueries
• Using wrong columns in subqueries