LEFT JOIN and RIGHT JOIN in PostgreSQL

Introduction

LEFT JOIN and RIGHT JOIN help you combine rows from two tables based on a related column, even if some rows don't have matching data in the other table.

You want to list all customers and their orders, including customers who have not placed any orders yet.

You need to find all employees and their departments, even if some employees are not assigned to any department.

You want to see all products and their sales, including products that have never been sold.

You want to compare two lists and keep all items from one list, showing related info if available.

Syntax

PostgreSQL

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

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

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

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

Examples

Shows all customers and their orders. Customers without orders still appear with NULL for order id.

PostgreSQL

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

Shows all departments and their employees. Departments without employees still appear with NULL for employee name.

PostgreSQL

SELECT employees.name, departments.name
FROM employees
RIGHT JOIN departments ON employees.department_id = departments.id;

Sample Program

This example creates two tables: customers and orders. It inserts some data, then shows how LEFT JOIN lists all customers with their orders (or NULL if none), and RIGHT JOIN lists all orders with their customers (or NULL if none).

PostgreSQL

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

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

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

-- RIGHT JOIN example
SELECT customers.name AS customer, orders.id AS order_id
FROM customers
RIGHT JOIN orders ON customers.id = orders.customer_id
ORDER BY orders.id;

Important Notes

LEFT JOIN and RIGHT JOIN are just mirror images; you can usually rewrite a RIGHT JOIN as a LEFT JOIN by switching table order.

If you only want rows where both tables match, use INNER JOIN instead.

NULL values in joined columns mean no matching row was found in the other table.

Summary

LEFT JOIN keeps all rows from the left table, adding matching rows from the right table or NULLs if no match.

RIGHT JOIN keeps all rows from the right table, adding matching rows from the left table or NULLs if no match.

Use these joins to include unmatched rows from one side when combining tables.

Practice

1. What does a LEFT JOIN do in PostgreSQL?

easy

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

B. Returns all rows from the right table and matching rows from the left table, or NULL if no match.

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

D. Returns all rows from both tables, matching where possible.

Solution

1. Step 1: Understand LEFT JOIN behavior

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

2. Step 2: Check what happens with unmatched rows

   If there is no matching row in the right table, the result includes NULLs for right table columns.

3. Final Answer:

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

4. Quick Check:

   LEFT JOIN = all left rows + matched right rows [OK]

Hint: LEFT JOIN keeps all left rows, fills right with NULL if no match [OK]

Common Mistakes:

• Confusing LEFT JOIN with RIGHT JOIN
• Thinking LEFT JOIN returns only matched rows
• Assuming unmatched rows are dropped

2. Which of the following is the correct syntax for a RIGHT JOIN in PostgreSQL?

easy

A. SELECT * FROM table1 RIGHT JOIN table2 ON table1.id = table2.id;

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

C. SELECT * FROM table1 RIGHT OUTER JOIN table2 WHERE table1.id = table2.id;

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

Solution

1. Step 1: Recall correct RIGHT JOIN syntax

   The correct syntax is: SELECT ... FROM left_table RIGHT JOIN right_table ON condition;

2. Step 2: Check each option

   SELECT * FROM table1 RIGHT JOIN table2 ON table1.id = table2.id; matches the correct syntax exactly. Options B, C, and D have syntax errors or misplaced keywords.

3. Final Answer:

   SELECT * FROM table1 RIGHT JOIN table2 ON table1.id = table2.id; -> Option A

4. Quick Check:

   RIGHT JOIN syntax = FROM left_table RIGHT JOIN right_table ON condition [OK]

Hint: RIGHT JOIN syntax: FROM left_table RIGHT JOIN right_table ON condition [OK]

Common Mistakes:

• Placing JOIN keyword incorrectly
• Using WHERE instead of ON for join condition
• Misordering table names

3. Given these tables:

employees
id | name
1 | Alice
2 | Bob
3 | Carol

departments
id | dept_name
2 | Sales
3 | HR
4 | IT

What is the result of this query?

SELECT employees.name, departments.dept_name FROM employees LEFT JOIN departments ON employees.id = departments.id ORDER BY employees.id;

medium

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

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

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

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

Solution

1. Step 1: Understand LEFT JOIN with given tables

   LEFT JOIN keeps all employees. For each employee, it tries to find a department with matching id.

2. Step 2: Match rows and fill NULLs

   Alice (id=1) has no matching department, so dept_name is NULL. Bob (id=2) matches Sales. Carol (id=3) matches HR.

3. Final Answer:

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

4. Quick Check:

   LEFT JOIN keeps all employees, unmatched dept_name is NULL [OK]

Hint: LEFT JOIN keeps all left rows; unmatched right columns are NULL [OK]

Common Mistakes:

• Assuming unmatched rows are excluded
• Mixing up LEFT and RIGHT JOIN results
• Ignoring NULL values for unmatched rows

4. You wrote this query but it gives an error:

SELECT * FROM employees RIGHT JOIN departments employees.id = departments.id;

What is the error and how to fix it?

medium

A. RIGHT JOIN is not supported; use LEFT JOIN instead

B. Missing ON keyword before join condition; fix by adding ON before employees.id = departments.id

C. Table names are reversed; switch employees and departments

D. Use WHERE instead of ON for join condition

Solution

1. Step 1: Identify syntax error in JOIN clause

   The query is missing the ON keyword before the join condition, which is required syntax.

2. Step 2: Correct the syntax

   Add ON before employees.id = departments.id to fix the error.

3. Final Answer:

   Missing ON keyword before join condition; fix by adding ON before employees.id = departments.id -> Option B

4. Quick Check:

   JOIN condition must use ON keyword [OK]

Hint: JOIN conditions always need ON keyword before condition [OK]

Common Mistakes:

• Omitting ON keyword
• Using WHERE instead of ON for join condition
• Confusing table order in JOIN

5. You want to list all departments and their employees, including departments with no employees. Which query achieves this?

hard

A. SELECT departments.dept_name, employees.name FROM employees RIGHT JOIN departments WHERE employees.id = departments.id;

B. SELECT departments.dept_name, employees.name FROM employees LEFT JOIN departments ON employees.id = departments.id;

C. SELECT departments.dept_name, employees.name FROM departments RIGHT JOIN employees ON departments.id = employees.id;

D. SELECT departments.dept_name, employees.name FROM departments LEFT JOIN employees ON departments.id = employees.id;

Solution

1. Step 1: Identify which table to keep all rows from

   We want all departments, even those without employees, so departments is the left table in LEFT JOIN.

2. Step 2: Use LEFT JOIN from departments to employees

   LEFT JOIN keeps all departments and adds matching employees or NULL if none.

3. Final Answer:

   SELECT departments.dept_name, employees.name FROM departments LEFT JOIN employees ON departments.id = employees.id; -> Option D

4. Quick Check:

   LEFT JOIN from departments keeps all departments [OK]

Hint: LEFT JOIN from main table keeps all its rows, unmatched get NULLs [OK]

Common Mistakes:

• Using employees as left table instead of departments
• Using RIGHT JOIN incorrectly
• Confusing which table to keep all rows from