Subquery in WHERE clause in SQL

Introduction

Subqueries in the WHERE clause help you filter data by using the result of another query. This lets you find rows that match conditions based on other data.

You want to find customers who have placed orders.

You need to list products that are in a specific category found in another table.

You want to get employees who work in departments located in a certain city.

You want to filter records based on values calculated or selected from another table.

Syntax

SQL

SELECT column1, column2
FROM table1
WHERE column3 operator (SELECT columnX FROM table2 WHERE condition);

The subquery inside the parentheses runs first and returns a value or list of values.

The main query uses this result to filter its rows.

Examples

This finds customers who have made at least one order.

SQL

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

This finds products in the 'Beverages' category.

SQL

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

This finds employees working in departments located in New York.

SQL

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.

SQL

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 can return one or many values. Use IN for multiple values, = for a single value.

Make sure the subquery returns compatible data types for comparison.

Summary

Subqueries in WHERE let you filter rows based on another query's results.

Use IN when the subquery returns multiple values, = when it returns one.

This helps connect data from different tables easily.

Practice

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

easy

A. Creates a new table from existing data

B. Filters rows based on the results of another query

C. Deletes rows from a table

D. Updates values in 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 results of another query.

2. Step 2: Compare with other SQL operations

   Creating tables, deleting, or updating rows are different SQL operations and not the purpose of subqueries in WHERE.

3. Final Answer:

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

4. Quick Check:

   Subquery in WHERE = filter rows [OK]

Hint: Subquery in WHERE filters rows using another query's results [OK]

Common Mistakes:

• Thinking subquery creates or modifies tables
• Confusing subquery with JOIN
• Assuming subquery always returns one value

2. Which of the following is the correct syntax to use a subquery in the WHERE clause to find employees in departments with ID 10 or 20?

easy

A. SELECT * FROM employees WHERE department_id IN SELECT department_id FROM departments WHERE department_id IN (10, 20);

B. SELECT * FROM employees WHERE department_id = (SELECT department_id FROM departments WHERE department_id IN (10, 20));

C. SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id = 10 OR 20);

D. SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id IN (10, 20));

Solution

1. Step 1: Identify correct subquery syntax with IN

   The subquery must be enclosed in parentheses and used with IN to match multiple values.

2. Step 2: Check each option's syntax

   SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id IN (10, 20)); correctly uses IN with a subquery in parentheses. SELECT * FROM employees WHERE department_id = (SELECT department_id FROM departments WHERE department_id IN (10, 20)); uses = which expects one value, causing error. SELECT * FROM employees WHERE department_id IN SELECT department_id FROM departments WHERE department_id IN (10, 20); misses parentheses around subquery. SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id = 10 OR 20); has incorrect WHERE clause syntax.

3. Final Answer:

   SELECT * FROM employees WHERE department_id IN (SELECT department_id FROM departments WHERE department_id IN (10, 20)); -> Option D

4. Quick Check:

   Subquery with IN needs parentheses [OK]

Hint: Use IN with parentheses for subqueries returning multiple values [OK]

Common Mistakes:

• Using = instead of IN for multiple values
• Omitting parentheses around subquery
• Incorrect WHERE clause conditions inside subquery

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

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

medium

A. Names of employees who work in the Sales department

B. Names of all employees

C. Names of departments named Sales

D. An error because subquery returns multiple rows

Solution

1. Step 1: Understand the subquery

   The subquery selects department_id from departments where name is 'Sales'. This returns IDs of Sales departments.

2. Step 2: Apply subquery results in main query

   The main query selects employee names where their department_id matches any of those returned by the subquery.

3. Final Answer:

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

4. Quick Check:

   Subquery filters employees by Sales department [OK]

Hint: Subquery filters department IDs, main query filters employees [OK]

Common Mistakes:

• Thinking subquery returns employee names
• Confusing department names with employee names
• Assuming subquery causes error with multiple rows

4. Identify the error in this query:

SELECT * FROM orders WHERE customer_id = (SELECT customer_id FROM customers WHERE city = 'New York');

medium

A. Incorrect table name 'orders'

B. Missing FROM clause in subquery

C. Subquery returns multiple rows causing an error with '=' operator

D. No error, query is correct

Solution

1. Step 1: Analyze subquery result

   The subquery selects customer_id from customers where city is 'New York'. This can return multiple customer IDs.

2. Step 2: Check operator compatibility

   The main query uses '=' which expects a single value, but subquery returns multiple rows, causing an error.

3. Final Answer:

   Subquery returns multiple rows causing an error with '=' operator -> Option C

4. Quick Check:

   Use IN for multiple subquery results [OK]

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

Common Mistakes:

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

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

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 NOT = (SELECT product_id FROM orders);

Solution

1. Step 1: Understand the goal

   We want products that have never been ordered, so their product_id should NOT appear in orders.

2. Step 2: Use NOT IN with subquery

   The subquery selects all product_ids from orders. Using NOT IN filters products not in that list.

3. Step 3: Check other options

   SELECT name FROM products WHERE product_id IN (SELECT product_id FROM orders); finds products that have been ordered (opposite). SELECT name FROM products WHERE product_id = (SELECT product_id FROM orders); uses = which expects one value, causing error. SELECT name FROM products WHERE product_id NOT = (SELECT product_id FROM orders); uses NOT = which is invalid syntax for multiple values.

4. Final Answer:

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

5. Quick Check:

   NOT IN filters products never ordered [OK]

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

Common Mistakes:

• Using = instead of IN for multiple values
• Confusing NOT IN with NOT =
• Selecting wrong table columns in subquery