Finding unmatched rows with LEFT JOIN in SQL - Step-by-Step Execution

Concept Flow - Finding unmatched rows with LEFT JOIN

Start with LEFT JOIN

↓

Match rows from left table

↓

Check if right table row exists

Yes No↓

Keep combined row

↓

Result set

LEFT JOIN keeps all rows from the left table and matches rows from the right table. If no match, right side columns are NULL.

Execution Sample

SQL

SELECT A.id, B.id
FROM A
LEFT JOIN B ON A.id = B.a_id
WHERE B.a_id IS NULL;

This query finds rows in table A that have no matching row in table B.

Execution Table

Step	A.id	B.a_id	B.id	Join Condition (A.id = B.a_id)	Action	Output Row Included?
1	1	1	10	1=1 True	Match found, combine rows	Yes
2	2	NULL	NULL	No matching B row	No match, keep A row with NULLs	Yes
3	3	3	30	3=3 True	Match found, combine rows	Yes
4	4	NULL	NULL	No matching B row	No match, keep A row with NULLs	Yes
5	Filter WHERE B.a_id IS NULL				Keep only rows where B.a_id is NULL	Rows with A.id 2 and 4

💡 All rows from A processed; final filter keeps only unmatched rows where B.a_id is NULL

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
A.id	N/A	1	2	3	4	N/A
B.a_id	N/A	1	NULL	3	NULL	Filtered NULL only
Output Rows	Empty	Row with A.id=1 and B.id=10	Row with A.id=2 and B.id=NULL	Row with A.id=3 and B.id=30	Row with A.id=4 and B.id=NULL	Rows with A.id=2 and 4 only

Key Moments - 2 Insights

Why do some rows have NULL values in the right table columns after LEFT JOIN?

Because those left table rows have no matching row in the right table, so SQL fills right side columns with NULLs (see execution_table rows 2 and 4).

Why do we use WHERE B.a_id IS NULL to find unmatched rows?

Because unmatched rows have NULL in right table columns, filtering on B.a_id IS NULL keeps only those unmatched rows (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of B.a_id at step 2?

A 1

B NULL

C 2

D 10

💡 Hint

Check the 'B.a_id' column in execution_table row 2

At which step does the join condition fail, resulting in NULLs for right table columns?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look for 'No matching B row' in the 'Action' column of execution_table

If we remove the WHERE clause filtering B.a_id IS NULL, what rows will the output include?

A All rows from A with matched or unmatched B rows

B Only rows without matches in B

C Only rows with matches in B

D No rows at all

💡 Hint

LEFT JOIN keeps all rows from A regardless of matches; WHERE clause filters unmatched rows

Concept Snapshot

LEFT JOIN returns all rows from the left table.
If no match in right table, right columns are NULL.
Use WHERE right_table.key IS NULL to find unmatched rows.
This helps find rows in left table without matches in right table.

Full Transcript

This visual execution shows how LEFT JOIN works to find unmatched rows. We start by joining tables A and B on matching IDs. For each row in A, SQL tries to find matching rows in B. If a match exists, it combines the rows. If not, it keeps the A row and fills B columns with NULL. Then, filtering WHERE B.a_id IS NULL keeps only those unmatched rows from A. The execution table traces each step, showing variable values and actions. This helps beginners see why NULLs appear and how filtering finds unmatched rows.

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