Subquery with EXISTS operator in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

SQL

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_date

Ccustomer_id

Dorder_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong column like order_id or order_date in the WHERE clause.

Not using the EXISTS operator properly.

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

2fill in blank

medium

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

SQL

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'

Aquantity

Border_id

Cid

Dproduct_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong column like order_id or quantity in the WHERE clause.

Forgetting to use NOT EXISTS to find products never ordered.

The subquery checks if there exists any order item with the same product_id as in the outer query. To find products never ordered, we use NOT EXISTS and compare product_id.

3fill in blank

hard

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

SQL

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'

Aemployee_id

Bdept_id

Cmanager_id

Ddepartment_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using manager_id from employees which may not exist.

Using department-related columns instead of employee ID.

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 orders with total amount greater than 100.

SQL

SELECT customer_id FROM customers WHERE EXISTS (SELECT 1 FROM orders WHERE orders.customer_id = customers.[1] AND orders.[2] > 100);

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

Acustomer_id

Border_id

Ctotal_amount

Dorder_date

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong columns like order_id or order_date for the amount comparison.

Not matching customer_id correctly.

The subquery checks for orders where the customer_id matches and the total_amount is greater than 100.

5fill in blank

hard

Fill all three blanks to select products that have been ordered more than 10 times in total.

SQL

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

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

Aproduct_id

Cquantity

Dorder_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using order_id instead of product_id in GROUP BY.

Using wrong column for summing like order_id.

The subquery groups order items by product_id and sums the quantity ordered. It checks if the total quantity is greater than 10 for the product matching the outer query.

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