LEFT JOIN with NULL result rows in SQL

Introduction

LEFT JOIN helps you combine two tables and keep all rows from the first table, even if there is no matching data in the second table. This shows missing matches as NULL.

You want to list all customers and their orders, even if some customers have no orders.

You need to find employees and their assigned projects, including those without projects.

You want to show all products and their sales, even if some products have zero sales.

You want to compare two lists and see which items are missing in the second list.

Syntax

SQL

SELECT columns
FROM table1
LEFT JOIN table2 ON table1.common_column = table2.common_column;

The LEFT JOIN keeps all rows from table1.

If there is no match in table2, columns from table2 will be NULL.

Examples

Shows all customers and their orders. Customers without orders show NULL for order id.

SQL

SELECT customers.name, orders.id
FROM customers
LEFT JOIN orders ON customers.id = orders.customer_id;

Lists all employees and their projects. Employees without projects show NULL for project name.

SQL

SELECT employees.name, projects.name
FROM employees
LEFT JOIN projects ON employees.project_id = projects.id;

Sample Program

This query lists all customers and their order IDs. Customers without orders show NULL in order_id.

SQL

CREATE TABLE customers (id INT, name VARCHAR(20));
CREATE TABLE orders (id INT, customer_id INT);

INSERT INTO customers VALUES (1, 'Alice'), (2, 'Bob'), (3, 'Carol');
INSERT INTO orders VALUES (101, 1), (102, 1), (103, 3);

SELECT customers.name, orders.id AS order_id
FROM customers
LEFT JOIN orders ON customers.id = orders.customer_id
ORDER BY customers.id, orders.id;

Important Notes

LEFT JOIN is useful to find unmatched rows by checking for NULLs in the joined table's columns.

Ordering results helps see NULL rows clearly at the right place.

Summary

LEFT JOIN keeps all rows from the first table.

Rows without matches in the second table show NULL values.

This helps find missing or unmatched data easily.

Practice

1. What does a LEFT JOIN do in SQL?

easy

A. Returns all rows from the left table and matched rows from the right table, NULL if no match.

B. Returns only rows that have matching values in both tables.

C. Returns all rows from the right table and matched rows from the left table.

D. Deletes rows from the left table that have no match in the right table.

Solution

1. Step 1: Understand LEFT JOIN behavior

   A LEFT JOIN keeps all rows from the left table regardless of matches in the right table.

2. Step 2: Identify NULLs for unmatched rows

   If there is no matching row in the right table, the result shows NULL for right table columns.

3. Final Answer:

   Returns all rows from the left table and matched rows from the right table, NULL if no match. -> Option A

4. Quick Check:

   LEFT JOIN = all left rows + NULL for no match [OK]

Hint: LEFT JOIN keeps all left rows, unmatched right rows show NULL [OK]

Common Mistakes:

• Confusing LEFT JOIN with INNER JOIN
• Thinking unmatched rows are dropped
• Assuming NULLs appear in left table columns

2. Which of the following is the correct syntax for a LEFT JOIN in SQL?

easy

A. SELECT * FROM table1 LEFT OUTER JOIN table2 WHERE table1.id = table2.id;

B. SELECT * FROM table1 JOIN LEFT table2 ON table1.id = table2.id;

C. SELECT * FROM table1 LEFT JOIN table2 USING (id);

D. SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id;

Solution

1. Step 1: Review standard LEFT JOIN syntax

   The correct syntax is: SELECT columns FROM left_table LEFT JOIN right_table ON condition.

2. Step 2: Check each option

   SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id; matches the correct syntax exactly. SELECT * FROM table1 JOIN LEFT table2 ON table1.id = table2.id; has JOIN LEFT which is invalid. SELECT * FROM table1 LEFT OUTER JOIN table2 WHERE table1.id = table2.id; is invalid because JOIN requires an ON clause (syntax error). SELECT * FROM table1 LEFT JOIN table2 USING (id); is correct syntax with parentheses.

3. Final Answer:

   SELECT * FROM table1 LEFT JOIN table2 ON table1.id = table2.id; -> Option D

4. Quick Check:

   LEFT JOIN syntax = LEFT JOIN ... ON ... [OK]

Hint: Use LEFT JOIN ... ON ... for correct syntax [OK]

Common Mistakes:

• Swapping JOIN and LEFT keywords
• Using WHERE instead of ON for join condition
• Omitting parentheses in USING clause

3. Given tables Employees and Departments with data:

Employees:
id | name | dept_id
1 | Alice | 10
2 | Bob | 20
3 | Carol | NULL

Departments:
dept_id | dept_name
10 | Sales
20 | HR
30 | IT

What is the result of this query?

SELECT e.name, d.dept_name FROM Employees e LEFT JOIN Departments d ON e.dept_id = d.dept_id;

medium

A. [{"name": "Alice", "dept_name": "Sales"}, {"name": "Bob", "dept_name": "HR"}]

B. [{"name": "Alice", "dept_name": "Sales"}, {"name": "Bob", "dept_name": "HR"}, {"name": "Carol", "dept_name": null}]

C. [{"name": "Alice", "dept_name": "Sales"}, {"name": "Bob", "dept_name": "HR"}, {"name": "Carol", "dept_name": "IT"}]

D. [{"name": "Alice", "dept_name": null}, {"name": "Bob", "dept_name": null}, {"name": "Carol", "dept_name": null}]

Solution

1. Step 1: Match Employees with Departments by dept_id

   Alice's dept_id 10 matches Sales, Bob's 20 matches HR, Carol's NULL has no match.

2. Step 2: Apply LEFT JOIN behavior

   All employees appear. For Carol, no matching department, so dept_name is NULL.

3. Final Answer:

   [{"name": "Alice", "dept_name": "Sales"}, {"name": "Bob", "dept_name": "HR"}, {"name": "Carol", "dept_name": null}] -> Option B

4. Quick Check:

   LEFT JOIN keeps all left rows, unmatched right columns NULL [OK]

Hint: LEFT JOIN shows NULL for unmatched right table rows [OK]

Common Mistakes:

• Omitting rows with NULL join keys
• Assuming unmatched rows get default values
• Confusing INNER JOIN output with LEFT JOIN

4. Consider this SQL query:

SELECT a.id, b.value FROM A a LEFT JOIN B b ON a.id = b.a_id WHERE b.value > 10;

Why might this query return fewer rows than table A has?

medium

A. Because the query syntax is invalid and causes an error.

B. Because LEFT JOIN only returns rows with matching b.value > 10.

C. Because the WHERE clause filters out rows where b.value is NULL, removing unmatched rows.

D. Because the ON condition is incorrect and causes no matches.

Solution

1. Step 1: Understand LEFT JOIN with WHERE filter

   LEFT JOIN keeps all rows from A, but WHERE filters after join.

2. Step 2: Effect of WHERE on NULLs from unmatched rows

   Rows with no match have b.value as NULL, and WHERE b.value > 10 excludes NULLs, removing those rows.

3. Final Answer:

   Because the WHERE clause filters out rows where b.value is NULL, removing unmatched rows. -> Option C

4. Quick Check:

   WHERE filters NULLs after LEFT JOIN, reducing rows [OK]

Hint: WHERE on right table column after LEFT JOIN filters out NULLs [OK]

Common Mistakes:

• Thinking LEFT JOIN always keeps all left rows regardless of WHERE
• Confusing ON and WHERE filtering effects
• Assuming query syntax error causes fewer rows

5. You have two tables:

Orders:
order_id | customer_id
1 | 101
2 | 102
3 | 103

Customers:
customer_id | name
101 | John
102 | Jane

You want to list all orders with customer names, but show 'Unknown' if no customer found.

Which SQL query correctly achieves this?

hard

A. SELECT o.order_id, COALESCE(c.name, 'Unknown') AS customer_name FROM Orders o LEFT JOIN Customers c ON o.customer_id = c.customer_id;

B. SELECT o.order_id, IFNULL(c.name, 'Unknown') AS customer_name FROM Orders o INNER JOIN Customers c ON o.customer_id = c.customer_id;

C. SELECT o.order_id, c.name FROM Orders o RIGHT JOIN Customers c ON o.customer_id = c.customer_id;

D. SELECT o.order_id, CASE WHEN c.name IS NULL THEN 'Unknown' ELSE c.name END FROM Orders o JOIN Customers c ON o.customer_id = c.customer_id;

Solution

1. Step 1: Use LEFT JOIN to keep all orders

   LEFT JOIN keeps all orders even if no matching customer exists.

2. Step 2: Replace NULL customer names with 'Unknown'

   Use COALESCE to show 'Unknown' when c.name is NULL.

3. Final Answer:

   SELECT o.order_id, COALESCE(c.name, 'Unknown') AS customer_name FROM Orders o LEFT JOIN Customers c ON o.customer_id = c.customer_id; -> Option A

4. Quick Check:

   LEFT JOIN + COALESCE handles missing customers [OK]

Hint: Use LEFT JOIN with COALESCE to replace NULLs [OK]

Common Mistakes:

• Using INNER JOIN excludes orders without customers
• Using RIGHT JOIN reverses table roles incorrectly
• Forgetting to handle NULL customer names