Subqueries with EXISTS in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all customers who have at least one order.

PostgreSQL

SELECT customer_id FROM customers WHERE EXISTS (SELECT 1 FROM orders WHERE orders.customer_id = customers.[1]);

Drag options to blanks, or click blank then click option'

Aid

Border_id

Ccustomer_name

Dcustomer_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong column like order_id or customer_name in the WHERE clause.

Not matching the correct customer_id column.

The subquery checks if there is any order with the same customer_id as the customer. So, the correct column to compare is customer_id.

2fill in blank

medium

Complete the code to find products that have never been ordered.

PostgreSQL

SELECT product_id FROM products WHERE NOT EXISTS (SELECT 1 FROM order_items WHERE order_items.product_id = products.[1]);

Drag options to blanks, or click blank then click option'

Aproduct_id

Bname

Corder_id

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong column like id or name in the WHERE clause.

Forgetting to use NOT EXISTS to find products never ordered.

The subquery checks if there is any order item with the same product_id as the product. To find products never ordered, we use NOT EXISTS and match on product_id.

3fill in blank

hard

Fix the error in the query to select employees who manage at least one department.

PostgreSQL

SELECT employee_id FROM employees WHERE EXISTS (SELECT 1 FROM departments WHERE departments.manager_id = employees.[1]);

Drag options to blanks, or click blank then click option'

Aid

Bemployee_id

Cmanager_id

Ddept_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong column like id or manager_id from employees.

Confusing columns from different tables.

The subquery checks if the manager_id in departments matches the employee_id in employees. So, the correct column to compare in employees is employee_id.

4fill in blank

hard

Fill both blanks to select customers who have placed orders with total amount greater than 1000.

PostgreSQL

SELECT customer_id FROM customers WHERE EXISTS (SELECT 1 FROM orders WHERE orders.customer_id = customers.[1] AND orders.[2] > 1000);

Drag options to blanks, or click blank then click option'

Acustomer_id

Border_id

Ctotal_amount

Damount

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong columns like order_id or amount instead of total_amount.

Not matching customer_id correctly.

The subquery checks orders where the customer_id matches the customer and the total_amount is greater than 1000.

5fill in blank

hard

Fill all three blanks to select products that have total ordered quantity greater than 10.

PostgreSQL

SELECT product_id FROM products WHERE EXISTS (SELECT 1 FROM order_items WHERE order_items.[1] = products.[2] GROUP BY order_items.[3] HAVING SUM(quantity) > 10);

Drag options to blanks, or click blank then click option'

Aproduct_id

Bid

Dorder_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong columns for matching or grouping.

Confusing product_id and id columns.

The subquery matches order_items.product_id to products.product_id, groups by order_items.product_id, and filters groups with total quantity over 10.

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