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.

Practice

1. What does a LEFT JOIN combined with WHERE right_table.key IS NULL do in SQL?

easy

A. Finds rows in the right table that have no matching rows in the left table

B. Finds rows in the left table that have no matching rows in the right table

C. Returns all rows from both tables regardless of matches

D. Deletes unmatched rows from the left table

Solution

1. Step 1: Understand LEFT JOIN behavior

   A LEFT JOIN returns all rows from the left table and matching rows from the right table. If no match exists, right table columns are NULL.

2. Step 2: Apply WHERE condition to filter unmatched rows

   Filtering with WHERE right_table.key IS NULL selects only those left table rows without a match in the right table.

3. Final Answer:

   Finds rows in the left table that have no matching rows in the right table -> Option B

4. Quick Check:

   LEFT JOIN + IS NULL = unmatched left rows [OK]

Hint: LEFT JOIN + IS NULL filters unmatched left table rows [OK]

Common Mistakes:

• Confusing unmatched rows as from right table
• Using INNER JOIN instead of LEFT JOIN
• Not checking for NULL in right table columns

2. Which of the following SQL queries correctly finds customers without orders using LEFT JOIN?

easy

A. SELECT c.id FROM customers c RIGHT JOIN orders o ON c.id = o.customer_id WHERE c.id IS NULL;

B. SELECT c.id FROM customers c INNER JOIN orders o ON c.id = o.customer_id WHERE o.customer_id IS NULL;

C. SELECT c.id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id WHERE c.id IS NULL;

D. SELECT c.id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id WHERE o.customer_id IS NULL;

Solution

1. Step 1: Identify correct JOIN type

   LEFT JOIN keeps all customers and matches orders; unmatched orders will be NULL.

2. Step 2: Filter unmatched orders

   WHERE o.customer_id IS NULL selects customers without orders.

3. Final Answer:

   SELECT c.id FROM customers c LEFT JOIN orders o ON c.id = o.customer_id WHERE o.customer_id IS NULL; -> Option D

4. Quick Check:

   LEFT JOIN + right table NULL = unmatched left rows [OK]

Hint: Use LEFT JOIN and check right table key IS NULL [OK]

Common Mistakes:

• Using INNER JOIN which excludes unmatched rows
• Checking NULL on left table columns
• Using RIGHT JOIN incorrectly for this case

3. Given tables employees(id, name) and tasks(employee_id, task_name), what does this query return?

SELECT e.name FROM employees e LEFT JOIN tasks t ON e.id = t.employee_id WHERE t.employee_id IS NULL;

medium

A. Names of employees who have no tasks assigned

B. Names of employees who have at least one task

C. All employee names regardless of tasks

D. Names of tasks without employees

Solution

1. Step 1: Analyze LEFT JOIN and ON condition

   The query joins employees with tasks on employee ID, keeping all employees.

2. Step 2: Filter rows where tasks are missing

   WHERE t.employee_id IS NULL selects employees with no matching tasks.

3. Final Answer:

   Names of employees who have no tasks assigned -> Option A

4. Quick Check:

   LEFT JOIN + NULL in right table = unmatched left rows [OK]

Hint: LEFT JOIN + right key NULL = unmatched left rows [OK]

Common Mistakes:

• Thinking it returns employees with tasks
• Confusing NULL check on left table
• Assuming INNER JOIN behavior

4. Identify the error in this query intended to find products without sales:

SELECT p.product_id FROM products p LEFT JOIN sales s ON p.product_id = s.product_id WHERE p.product_id IS NULL;

medium

A. ON condition is incorrect, should join on sales_id

B. LEFT JOIN should be INNER JOIN

C. The WHERE clause should check s.product_id IS NULL, not p.product_id

D. Query is correct and will return unmatched products

Solution

1. Step 1: Understand LEFT JOIN result

   LEFT JOIN keeps all products; unmatched sales columns are NULL.

2. Step 2: Check WHERE clause correctness

   Filtering on p.product_id IS NULL is wrong because left table columns are never NULL in LEFT JOIN; should check s.product_id IS NULL.

3. Final Answer:

   The WHERE clause should check s.product_id IS NULL, not p.product_id -> Option C

4. Quick Check:

   Filter NULL on right table columns, not left [OK]

Hint: Check NULL on right table columns after LEFT JOIN [OK]

Common Mistakes:

• Checking NULL on left table columns
• Using INNER JOIN instead of LEFT JOIN
• Incorrect ON join condition

5. You have tables students(id, name) and enrollments(student_id, course_id). Write a query to find students not enrolled in any course, considering some students may have NULL IDs. Which query correctly handles this?

hard

A. SELECT s.name FROM students s LEFT JOIN enrollments e ON s.id = e.student_id WHERE e.student_id IS NULL AND s.id IS NOT NULL;

B. SELECT s.name FROM students s RIGHT JOIN enrollments e ON s.id = e.student_id WHERE s.id IS NULL;

C. SELECT s.name FROM students s INNER JOIN enrollments e ON s.id = e.student_id WHERE s.id IS NULL;

D. SELECT s.name FROM students s LEFT JOIN enrollments e ON s.id = e.student_id WHERE e.student_id IS NULL;

Solution

1. Step 1: Use LEFT JOIN to find unmatched students

   LEFT JOIN students with enrollments keeps all students; unmatched enrollments are NULL.

2. Step 2: Filter students without enrollments and exclude NULL student IDs

   WHERE e.student_id IS NULL finds students without courses; adding s.id IS NOT NULL excludes students with NULL IDs to avoid incorrect matches.

3. Final Answer:

   SELECT s.name FROM students s LEFT JOIN enrollments e ON s.id = e.student_id WHERE e.student_id IS NULL AND s.id IS NOT NULL; -> Option A

4. Quick Check:

   LEFT JOIN + right NULL + exclude NULL left keys = correct unmatched [OK]

Hint: Exclude NULL keys on left table when filtering unmatched [OK]

Common Mistakes:

• Ignoring NULL IDs in left table
• Using INNER JOIN which excludes unmatched rows
• Using RIGHT JOIN incorrectly