Subquery in WHERE clause in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a subquery in the WHERE clause?

A subquery in the WHERE clause is a query inside another query that helps filter results based on values returned by the inner query.

beginner

How does a subquery in the WHERE clause work?

The outer query uses the result of the subquery to decide which rows to include. The subquery runs first and returns values that the outer query compares against.

beginner

Example: SELECT * FROM employees WHERE department_id IN (SELECT id FROM departments WHERE location = 'New York');
What does this query do?

It finds all employees who work in departments located in New York. The subquery finds department IDs in New York, and the outer query selects employees in those departments.

intermediate

Can a subquery in the WHERE clause return multiple values?

Yes, subqueries can return multiple values when used with operators like IN, ANY, or ALL to compare against multiple results.

intermediate

What is the difference between using '=' and 'IN' with a subquery in the WHERE clause?

'=' expects the subquery to return a single value, while 'IN' allows the subquery to return multiple values to match any of them.

What does a subquery in the WHERE clause do?

A Creates a new table

B Filters rows based on values returned by the inner query

C Updates data in the database

D Deletes rows from a table

A subquery in the WHERE clause filters rows by using values returned from the inner query.

Which operator allows a subquery to return multiple values in the WHERE clause?

A =

B BETWEEN

C IN

D LIKE

The IN operator allows the subquery to return multiple values to match any of them.

What happens if a subquery in the WHERE clause returns no rows?

A The outer query returns no rows

B The outer query returns all rows

C The database throws an error

D The subquery runs again

If the subquery returns no rows, the outer query finds no matching rows and returns no results.

Which of these is a valid use of a subquery in the WHERE clause?

A SELECT * FROM products WHERE price IN 'SELECT price FROM products';

B SELECT * FROM products WHERE price = 'SELECT AVG(price) FROM products';

C SELECT * FROM products WHERE price > ALL 'SELECT price FROM products';

D SELECT * FROM products WHERE price > (SELECT AVG(price) FROM products);

Option D correctly uses a subquery to compare price to the average price.

Can a subquery in the WHERE clause reference columns from the outer query?

A Yes, this is called a correlated subquery

B No, subqueries cannot reference outer query columns

C Only if the subquery is in the SELECT clause

D Only if the subquery uses JOIN

A correlated subquery references columns from the outer query and runs once per outer row.

Explain how a subquery in the WHERE clause helps filter data with an example.

Think about how the inner query returns values that the outer query uses to decide which rows to keep.

Describe the difference between using '=' and 'IN' with subqueries in the WHERE clause.

Consider what happens if the subquery returns one value or many values.

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