Finding unmatched rows with LEFT JOIN in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all customers who have no orders.

SQL

SELECT customers.id, customers.name FROM customers LEFT JOIN orders ON customers.id = orders.customer_id WHERE orders.customer_id IS [1];

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

AFALSE

BNOT NULL

CNULL

DTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using = NULL instead of IS NULL

Checking for NOT NULL instead of NULL

Using INNER JOIN instead of LEFT JOIN

When using LEFT JOIN, unmatched rows in the right table have NULL values. So, to find customers without orders, we check where orders.customer_id IS NULL.

2fill in blank

medium

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

SQL

SELECT products.id, products.name FROM products LEFT JOIN order_items ON products.id = order_items.product_id WHERE order_items.product_id IS [1];

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

ANOT NULL

BNULL

CFALSE

DTRUE

Attempts:

3 left

💡 Hint

Common Mistakes

Using INNER JOIN which excludes unmatched rows

Checking for NOT NULL instead of NULL

Using = NULL instead of IS NULL

Products with no matching order_items will have NULL in order_items.product_id after the LEFT JOIN.

3fill in blank

hard

Complete the code to find employees without assigned projects.

SQL

SELECT e.id, e.name FROM employees e LEFT JOIN projects p ON e.id = p.employee_id WHERE p.employee_id IS [1];

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

AFALSE

BNOT NULL

C0

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Using = NULL instead of IS NULL

Checking for 0 or FALSE which are not correct for NULL

Using INNER JOIN instead of LEFT JOIN

Employees with no matching projects will have NULL in p.employee_id after the LEFT JOIN. Use IS NULL to find them.

4fill in blank

hard

Fill both blanks to find customers without orders and select their id and name.

SQL

SELECT customers.[1], customers.[2] FROM customers LEFT JOIN orders ON customers.id = orders.customer_id WHERE orders.customer_id IS NULL;

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

Aid

Bname

Corder_id

Dcustomer_id

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting columns from the orders table instead of customers

Using wrong column names like order_id or customer_id from orders

We select the customer's id and name columns to identify customers without orders.

5fill in blank

hard

Fill all three blanks to find products without orders and select their id, name, and price.

SQL

SELECT [1], [2], [3] FROM products LEFT JOIN order_items ON products.id = order_items.product_id WHERE order_items.product_id IS NULL;

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

Aproducts.id

Bproducts.name

Cproducts.price

Dorder_items.quantity

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting columns from order_items instead of products

Missing one or more required columns

We select the product's id, name, and price to identify products without any orders.