Finding unmatched rows with LEFT JOIN in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a LEFT JOIN do in SQL?

A LEFT JOIN returns all rows from the left table and the matched rows from the right table. If there is no match, the result is NULL on the right side.

beginner

How can you find rows in the left table that have no matching rows in the right table using LEFT JOIN?

Use a LEFT JOIN and then filter where the right table's key column is NULL. This shows unmatched rows from the left table.

beginner

Why do we check for NULL in the right table's columns after a LEFT JOIN?

Because unmatched rows from the left table will have NULL values in the right table's columns, indicating no match was found.

intermediate

Write a simple SQL query to find customers who have no orders using LEFT JOIN.

SELECT customers.* FROM customers LEFT JOIN orders ON customers.id = orders.customer_id WHERE orders.customer_id IS NULL;

intermediate

What is the difference between LEFT JOIN and INNER JOIN when finding unmatched rows?

INNER JOIN returns only matching rows from both tables, so it excludes unmatched rows. LEFT JOIN returns all left table rows including unmatched ones.

What does the condition 'WHERE right_table.id IS NULL' do after a LEFT JOIN?

A Finds rows in the left table with no match in the right table

B Finds rows in the right table only

C Finds rows with matching keys in both tables

D Deletes unmatched rows

Checking for NULL in the right table's key column after a LEFT JOIN identifies rows in the left table that have no matching row in the right table.

Which JOIN type returns all rows from the left table regardless of matches?

A LEFT JOIN

B RIGHT JOIN

C FULL JOIN

D INNER JOIN

LEFT JOIN returns all rows from the left table and matched rows from the right table.

If you want to find unmatched rows in the right table, which JOIN would you use?

A LEFT JOIN

B RIGHT JOIN

C INNER JOIN

D CROSS JOIN

RIGHT JOIN returns all rows from the right table and matched rows from the left table. Checking for NULL in the left table columns finds unmatched right table rows.

What happens to unmatched rows from the right table in a LEFT JOIN?

A They are duplicated

B They are included with NULLs in left table columns

C They cause an error

D They are excluded from the result

LEFT JOIN includes all left table rows. Unmatched right table rows are excluded.

Which SQL clause is essential to filter unmatched rows after a LEFT JOIN?

A GROUP BY

B HAVING

C WHERE right_table.column IS NULL

D ORDER BY

Filtering with WHERE right_table.column IS NULL selects rows from the left table without matches in the right table.

Explain how to find unmatched rows in one table using LEFT JOIN.

Think about what happens to unmatched rows in a LEFT JOIN result.

Describe the difference between INNER JOIN and LEFT JOIN when looking for unmatched rows.

Consider which join keeps unmatched rows from the left table.

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