Subquery with EXISTS operator in SQL - Mini Project: Build & Apply

Using Subquery with EXISTS Operator

📖 Scenario: You work for a bookstore database. The database has two tables: Books and Orders. You want to find all books that have been ordered at least once.

🎯 Goal: Build a SQL query using the EXISTS operator with a subquery to list all books that have orders.

📋 What You'll Learn

Create a Books table with columns BookID (integer) and Title (text).

Create an Orders table with columns OrderID (integer) and BookID (integer).

Write a query using EXISTS with a subquery to select all books that have at least one order.

Use exact table and column names as specified.

💡 Why This Matters

🌍 Real World

Bookstores and many businesses use databases to track products and orders. Checking if a product has been ordered is a common task.

💼 Career

Understanding subqueries and EXISTS is important for database querying roles, data analysis, and backend development.

1

Create the Books table with sample data

Create a table called Books with columns BookID (integer) and Title (text). Insert these exact rows: (1, 'Learn SQL'), (2, 'Database Basics'), (3, 'Advanced SQL').

SQL

-- Create Books table and insert data
-- Your code here

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows.

2

Create the Orders table with sample data

Create a table called Orders with columns OrderID (integer) and BookID (integer). Insert these exact rows: (101, 1), (102, 3).

SQL

CREATE TABLE Books (BookID INTEGER, Title TEXT);
INSERT INTO Books (BookID, Title) VALUES (1, 'Learn SQL'), (2, 'Database Basics'), (3, 'Advanced SQL');
-- Create Orders table and insert data
-- Your code here

Hint

Use CREATE TABLE and INSERT INTO like in Step 1.

3

Write the query using EXISTS subquery

Write a SQL query to select BookID and Title from Books where there exists at least one row in Orders with the same BookID. Use the EXISTS operator with a subquery.

SQL

CREATE TABLE Books (BookID INTEGER, Title TEXT);
INSERT INTO Books (BookID, Title) VALUES (1, 'Learn SQL'), (2, 'Database Basics'), (3, 'Advanced SQL');
CREATE TABLE Orders (OrderID INTEGER, BookID INTEGER);
INSERT INTO Orders (OrderID, BookID) VALUES (101, 1), (102, 3);
-- Write SELECT query using EXISTS
-- Your code here

Hint

Use WHERE EXISTS (SELECT 1 FROM Orders WHERE Orders.BookID = Books.BookID) to check for matching orders.

4

Complete the query with ordering

Add an ORDER BY Title clause at the end of the query to sort the results alphabetically by book title.

SQL

CREATE TABLE Books (BookID INTEGER, Title TEXT);
INSERT INTO Books (BookID, Title) VALUES (1, 'Learn SQL'), (2, 'Database Basics'), (3, 'Advanced SQL');
CREATE TABLE Orders (OrderID INTEGER, BookID INTEGER);
INSERT INTO Orders (OrderID, BookID) VALUES (101, 1), (102, 3);
SELECT BookID, Title FROM Books WHERE EXISTS (SELECT 1 FROM Orders WHERE Orders.BookID = Books.BookID)
-- Add ORDER BY clause
-- Your code here

Hint

Use ORDER BY Title at the end of the query to sort results.

Practice

1. What does the EXISTS operator do in an SQL query?

easy

A. Checks if a subquery returns any rows and returns TRUE or FALSE.

B. Counts the number of rows in a table.

C. Joins two tables based on a condition.

D. Deletes rows from a table.

Solution

1. Step 1: Understand the purpose of EXISTS

   The EXISTS operator checks if the subquery returns at least one row.

2. Step 2: Compare with other options

   Counting rows, joining tables, or deleting rows are different SQL operations unrelated to EXISTS.

3. Final Answer:

   Checks if a subquery returns any rows and returns TRUE or FALSE. -> Option A

4. Quick Check:

   EXISTS = TRUE if rows found [OK]

Hint: EXISTS means "is there at least one row?" [OK]

Common Mistakes:

• Thinking EXISTS counts rows instead of checking existence
• Confusing EXISTS with JOIN operations
• Assuming EXISTS deletes or modifies data

2. Which of the following is the correct syntax to use EXISTS in a WHERE clause?

easy

A. SELECT * FROM table WHERE EXISTS (SELECT column FROM table2);

B. SELECT * FROM table WHERE EXISTS = (SELECT column FROM table2);

C. SELECT * FROM table WHERE EXISTS IN (SELECT column FROM table2);

D. SELECT * FROM table WHERE EXISTS LIKE (SELECT column FROM table2);

Solution

1. Step 1: Review correct EXISTS syntax

   EXISTS is used as EXISTS (subquery) without any operator like =, IN, or LIKE.

2. Step 2: Identify incorrect options

   Options B, C, and D misuse operators (=, IN, LIKE) with EXISTS, causing syntax errors.

3. Final Answer:

   SELECT * FROM table WHERE EXISTS (SELECT column FROM table2); -> Option A

4. Quick Check:

   EXISTS uses only parentheses for subquery [OK]

Hint: EXISTS always followed by (subquery) without operators [OK]

Common Mistakes:

• Adding = or IN after EXISTS
• Using LIKE with EXISTS
• Forgetting parentheses around subquery

3. Given tables Customers(id, name) and Orders(id, customer_id), what does this query return?

SELECT name FROM Customers c WHERE EXISTS (SELECT 1 FROM Orders o WHERE o.customer_id = c.id);

medium

A. All customers including those without orders.

B. All customers who have placed at least one order.

C. All orders with customer names.

D. An error because of incorrect subquery.

Solution

1. Step 1: Understand the EXISTS subquery condition

   The subquery checks if there is at least one order with customer_id matching the customer's id.

2. Step 2: Determine which customers are returned

   Only customers with matching orders exist, so only those customers' names are returned.

3. Final Answer:

   All customers who have placed at least one order. -> Option B

4. Quick Check:

   EXISTS filters customers with orders [OK]

Hint: EXISTS filters rows with matching related data [OK]

Common Mistakes:

• Thinking it returns all customers regardless of orders
• Confusing with JOIN that returns all orders
• Assuming syntax error due to subquery

4. Identify the error in this query:

SELECT * FROM Employees e WHERE EXISTS SELECT * FROM Salaries s WHERE s.emp_id = e.id;

medium

A. EXISTS cannot be used in WHERE clause.

B. Using SELECT * inside EXISTS is not allowed.

C. Missing parentheses around the subquery after EXISTS.

D. The alias 'e' is not defined.

Solution

1. Step 1: Check EXISTS syntax

   EXISTS requires the subquery to be enclosed in parentheses.

2. Step 2: Identify the missing parentheses

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

3. Final Answer:

   Missing parentheses around the subquery after EXISTS. -> Option C

4. Quick Check:

   EXISTS (subquery) needs parentheses [OK]

Hint: Always put parentheses around subquery after EXISTS [OK]

Common Mistakes:

• Omitting parentheses around subquery
• Thinking SELECT * is invalid inside EXISTS
• Misunderstanding alias usage

5. You want to find all products that have never been ordered. Given tables Products(id, name) and Orders(id, product_id), which query correctly uses EXISTS to find these products?

hard

A. SELECT name FROM Products p WHERE NOT IN (SELECT product_id FROM Orders);

B. SELECT name FROM Products p WHERE EXISTS (SELECT 1 FROM Orders o WHERE o.product_id = p.id);

C. SELECT name FROM Products p WHERE EXISTS NOT (SELECT 1 FROM Orders o WHERE o.product_id = p.id);

D. SELECT name FROM Products p WHERE NOT EXISTS (SELECT 1 FROM Orders o WHERE o.product_id = p.id);

Solution

1. Step 1: Understand the goal

   We want products with no matching orders, so the subquery should check for orders and we want those where no such orders exist.

2. Step 2: Use NOT EXISTS correctly

   NOT EXISTS with a subquery checking orders for the product returns products never ordered.

3. Step 3: Evaluate other options

   SELECT name FROM Products p WHERE EXISTS (SELECT 1 FROM Orders o WHERE o.product_id = p.id); returns products with orders, C has invalid syntax, D uses NOT IN which can cause issues with NULLs.

4. Final Answer:

   SELECT name FROM Products p WHERE NOT EXISTS (SELECT 1 FROM Orders o WHERE o.product_id = p.id); -> Option D

5. Quick Check:

   NOT EXISTS finds missing related rows [OK]

Hint: Use NOT EXISTS to find items with no related rows [OK]

Common Mistakes:

• Using EXISTS instead of NOT EXISTS for missing data
• Incorrect syntax like EXISTS NOT
• Using NOT IN which may fail with NULLs