Subqueries with EXISTS in PostgreSQL - Mini Project: Build & Apply

Find Customers with Orders Using EXISTS Subquery

📖 Scenario: You work at a small online store. You have two tables: customers and orders. You want to find which customers have placed at least one order.

🎯 Goal: Build a SQL query using a subquery with EXISTS to list all customers who have made orders.

📋 What You'll Learn

Create a customers table with columns customer_id and customer_name.

Create an orders table with columns order_id and customer_id.

Insert the exact customers and orders data as specified.

Write a SQL query using EXISTS to find customers with orders.

💡 Why This Matters

🌍 Real World

Stores and businesses often need to find customers who have made purchases to send promotions or analyze sales.

💼 Career

Knowing how to use EXISTS subqueries is important for database querying tasks in data analysis, backend development, and reporting.

1

Create the customers table and insert data

Write SQL statements to create a table called customers with columns customer_id (integer) and customer_name (text). Then insert these exact rows: (1, 'Alice'), (2, 'Bob'), (3, 'Charlie').

PostgreSQL

-- Create customers 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 and insert data

Write SQL statements to create a table called orders with columns order_id (integer) and customer_id (integer). Then insert these exact rows: (101, 1), (102, 1), (103, 3).

PostgreSQL

CREATE TABLE customers (
    customer_id INTEGER PRIMARY KEY,
    customer_name TEXT
);

INSERT INTO customers (customer_id, customer_name) VALUES
(1, 'Alice'),
(2, 'Bob'),
(3, 'Charlie');

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

Hint

Use CREATE TABLE and INSERT INTO like before, matching the exact columns and values.

3

Write the query using EXISTS to find customers with orders

Write a SQL query that selects customer_id and customer_name from customers where there exists at least one row in orders with the same customer_id. Use a subquery with EXISTS.

PostgreSQL

CREATE TABLE customers (
    customer_id INTEGER PRIMARY KEY,
    customer_name TEXT
);

INSERT INTO customers (customer_id, customer_name) VALUES
(1, 'Alice'),
(2, 'Bob'),
(3, 'Charlie');

CREATE TABLE orders (
    order_id INTEGER PRIMARY KEY,
    customer_id INTEGER
);

INSERT INTO orders (order_id, customer_id) VALUES
(101, 1),
(102, 1),
(103, 3);

-- Write SELECT query using EXISTS
-- Your code here

Hint

Use WHERE EXISTS (SELECT 1 FROM orders WHERE orders.customer_id = customers.customer_id) to check if the customer has any orders.

4

Complete the query with ordering by customer_name

Add an ORDER BY clause to the query to sort the results by customer_name in ascending order.

PostgreSQL

CREATE TABLE customers (
    customer_id INTEGER PRIMARY KEY,
    customer_name TEXT
);

INSERT INTO customers (customer_id, customer_name) VALUES
(1, 'Alice'),
(2, 'Bob'),
(3, 'Charlie');

CREATE TABLE orders (
    order_id INTEGER PRIMARY KEY,
    customer_id INTEGER
);

INSERT INTO orders (order_id, customer_id) VALUES
(101, 1),
(102, 1),
(103, 3);

SELECT customer_id, customer_name
FROM customers
WHERE EXISTS (
    SELECT 1 FROM orders WHERE orders.customer_id = customers.customer_id
);

-- Add ORDER BY customer_name
-- Your code here

Hint

Use ORDER BY customer_name ASC after the WHERE EXISTS clause.

Practice

1. What does the EXISTS keyword do in a SQL query?

easy

A. Checks if a subquery returns any rows and returns true or false

B. Counts the total 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 keyword is used to check if a subquery returns any rows. It returns true if there is at least one row, otherwise false.

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 subquery has rows [OK]

Hint: EXISTS means: 'Is there at least one row?' [OK]

Common Mistakes:

• Confusing EXISTS with COUNT
• Thinking EXISTS joins tables
• Assuming EXISTS deletes data

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

easy

A. WHERE EXISTS IN (SELECT * FROM orders WHERE customer_id = customers.id)

B. WHERE EXISTS SELECT * FROM orders WHERE customer_id = customers.id

C. WHERE EXISTS (SELECT * FROM orders WHERE customer_id = customers.id)

D. WHERE EXISTS = (SELECT * FROM orders WHERE customer_id = customers.id)

Solution

1. Step 1: Check correct EXISTS syntax

   The correct syntax requires EXISTS followed by a subquery enclosed in parentheses.

2. Step 2: Validate each option

   WHERE EXISTS (SELECT * FROM orders WHERE customer_id = customers.id) uses parentheses correctly. The other options have syntax errors or incorrect keywords.

3. Final Answer:

   WHERE EXISTS (SELECT * FROM orders WHERE customer_id = customers.id) -> Option C

4. Quick Check:

   EXISTS needs parentheses around subquery [OK]

Hint: EXISTS always followed by parentheses with a subquery [OK]

Common Mistakes:

• Omitting parentheses after EXISTS
• Using '=' or 'IN' with EXISTS
• Writing subquery outside parentheses

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

SELECT name FROM customers WHERE EXISTS (SELECT 1 FROM orders WHERE orders.customer_id = customers.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 invalid subquery

Solution

1. Step 1: Understand the EXISTS subquery

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

2. Step 2: Determine the main query result

   The main query selects customer names only if the subquery returns rows, meaning the customer has orders.

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 having matching related rows [OK]

Common Mistakes:

• Thinking it returns all customers
• Confusing with JOIN returning orders
• Assuming syntax error in subquery

4. Identify the error in this query:

SELECT id FROM customers WHERE EXISTS SELECT * FROM orders WHERE orders.customer_id = customers.id;

medium

A. Incorrect table alias usage

B. Using SELECT * instead of SELECT 1 in EXISTS

C. Missing WHERE clause in subquery

D. Missing parentheses around the subquery after EXISTS

Solution

1. Step 1: Check EXISTS syntax

   The EXISTS keyword must be followed by a subquery enclosed in parentheses.

2. Step 2: Identify the error in the query

   The query misses parentheses around the subquery, causing a syntax error.

3. Final Answer:

   Missing parentheses around the subquery after EXISTS -> Option D

4. Quick Check:

   EXISTS requires parentheses around subquery [OK]

Hint: Always put parentheses after EXISTS [OK]

Common Mistakes:

• Omitting parentheses after EXISTS
• Thinking SELECT * is invalid in EXISTS
• Confusing alias usage

5. You want to find all customers who have placed orders only in the last 30 days. Which query correctly uses EXISTS to achieve this?

A) SELECT name FROM customers WHERE EXISTS (SELECT 1 FROM orders WHERE orders.customer_id = customers.id AND order_date >= CURRENT_DATE - INTERVAL '30 days');
B) SELECT name FROM customers WHERE EXISTS (SELECT 1 FROM orders WHERE orders.customer_id = customers.id AND order_date <= CURRENT_DATE - INTERVAL '30 days');
C) SELECT name FROM customers WHERE EXISTS (SELECT 1 FROM orders WHERE orders.customer_id = customers.id);
D) SELECT name FROM customers WHERE EXISTS (SELECT 1 FROM orders WHERE order_date >= CURRENT_DATE - INTERVAL '30 days');

hard

A. Select customers with orders in last 30 days using correct date filter

B. Select customers with orders older than 30 days

C. Select customers with any orders regardless of date

D. Select customers if any order in last 30 days ignoring customer link

Solution

1. Step 1: Understand the date filter requirement

   We want customers with orders placed in the last 30 days, so the order date must be >= current date minus 30 days.

2. Step 2: Analyze each query option

   Select customers with orders in last 30 days using correct date filter correctly filters orders by customer and date. Select customers with orders older than 30 days filters orders older than 30 days. Select customers with any orders regardless of date ignores date. Select customers if any order in last 30 days ignoring customer link ignores customer link.

3. Final Answer:

   Select customers with orders in last 30 days using correct date filter -> Option A

4. Quick Check:

   EXISTS with date filter and customer match [OK]

Hint: Add date condition inside EXISTS subquery with customer match [OK]

Common Mistakes:

• Using wrong date comparison operator
• Forgetting to link orders to customers
• Filtering orders without customer condition