LEFT JOIN vs RIGHT JOIN decision in SQL - Hands-On Comparison

Understanding LEFT JOIN vs RIGHT JOIN in SQL

📖 Scenario: You work at a small bookstore. You have two tables: Books and Authors. Some books have authors listed, but some authors have not written any books yet. You want to learn how to use SQL joins to find all books with their authors, and also all authors with their books.

🎯 Goal: Build SQL queries step-by-step to understand the difference between LEFT JOIN and RIGHT JOIN. You will create tables, add data, and write queries that show all books with authors and all authors with books.

📋 What You'll Learn

Create two tables: Books and Authors

Insert specific data into both tables

Write a LEFT JOIN query to list all books with their authors

Write a RIGHT JOIN query to list all authors with their books

💡 Why This Matters

🌍 Real World

Joins are used in databases to combine related data from different tables, like matching books to authors.

💼 Career

Understanding LEFT JOIN and RIGHT JOIN is essential for database querying in roles like data analyst, backend developer, and database administrator.

1

Create the Books and Authors tables

Write SQL commands to create a table called Books with columns BookID (integer, primary key) and Title (text). Also create a table called Authors with columns AuthorID (integer, primary key) and Name (text).

SQL

-- Create Books and Authors tables
-- Your code here

Hint

Use CREATE TABLE statements with the exact column names and types.

2

Insert data into Books and Authors

Insert these rows into Books: (1, 'The Great Gatsby'), (2, '1984'), (4, 'Unknown Book'). Insert these rows into Authors: (1, 'F. Scott Fitzgerald'), (2, 'George Orwell'), (3, 'New Author').

SQL

CREATE TABLE Books (
  BookID INTEGER PRIMARY KEY,
  Title TEXT
);

CREATE TABLE Authors (
  AuthorID INTEGER PRIMARY KEY,
  Name TEXT
);

-- Insert data into Books and Authors
-- Your code here

Hint

Use INSERT INTO with the exact values given.

3

Write a LEFT JOIN query to list all books with their authors

Write a SQL query that selects Books.Title and Authors.Name using a LEFT JOIN on Books.BookID = Authors.AuthorID. This will show all books and their authors if available.

SQL

CREATE TABLE Books (
  BookID INTEGER PRIMARY KEY,
  Title TEXT
);

CREATE TABLE Authors (
  AuthorID INTEGER PRIMARY KEY,
  Name TEXT
);

INSERT INTO Books (BookID, Title) VALUES (1, 'The Great Gatsby'), (2, '1984'), (4, 'Unknown Book');
INSERT INTO Authors (AuthorID, Name) VALUES (1, 'F. Scott Fitzgerald'), (2, 'George Orwell'), (3, 'New Author');

-- Write LEFT JOIN query here
-- Your code here

Hint

Use LEFT JOIN to get all books, even if no author matches.

4

Write a RIGHT JOIN query to list all authors with their books

Write a SQL query that selects Books.Title and Authors.Name using a RIGHT JOIN on Books.BookID = Authors.AuthorID. This will show all authors and their books if available.

SQL

CREATE TABLE Books (
  BookID INTEGER PRIMARY KEY,
  Title TEXT
);

CREATE TABLE Authors (
  AuthorID INTEGER PRIMARY KEY,
  Name TEXT
);

INSERT INTO Books (BookID, Title) VALUES (1, 'The Great Gatsby'), (2, '1984'), (4, 'Unknown Book');
INSERT INTO Authors (AuthorID, Name) VALUES (1, 'F. Scott Fitzgerald'), (2, 'George Orwell'), (3, 'New Author');

SELECT Books.Title, Authors.Name
FROM Books
LEFT JOIN Authors ON Books.BookID = Authors.AuthorID;

-- Write RIGHT JOIN query here
-- Your code here

Hint

Use RIGHT JOIN to get all authors, even if no book matches.

Practice

1. Which JOIN type keeps all rows from the first (left) table, even if there is no matching row in the second (right) table?

easy

A. RIGHT JOIN

B. INNER JOIN

C. LEFT JOIN

D. FULL JOIN

Solution

1. Step 1: Understand LEFT JOIN behavior

   LEFT JOIN returns all rows from the first table and matching rows from the second table. If no match, NULLs appear for second table columns.

2. Step 2: Compare with RIGHT JOIN

   RIGHT JOIN keeps all rows from the second table, not the first. INNER JOIN only returns matching rows. FULL JOIN returns all rows from both tables.

3. Final Answer:

   LEFT JOIN -> Option C

4. Quick Check:

   LEFT JOIN keeps all left table rows [OK]

Hint: LEFT JOIN keeps all from first table, RIGHT JOIN from second [OK]

Common Mistakes:

• Confusing LEFT JOIN with RIGHT JOIN
• Thinking INNER JOIN keeps unmatched rows
• Assuming FULL JOIN keeps only one table's rows

2. Which of the following is the correct syntax to perform a RIGHT JOIN between tables Employees and Departments on Employees.DeptID = Departments.ID?

easy

A. SELECT * FROM Employees RIGHT JOIN Departments ON Employees.DeptID = Departments.ID;

B. SELECT * FROM Employees LEFT JOIN Departments ON Employees.DeptID = Departments.ID;

C. SELECT * FROM Employees RIGHT JOIN Departments WHERE Employees.DeptID = Departments.ID;

D. SELECT * FROM Employees JOIN Departments ON Employees.DeptID = Departments.ID;

Solution

1. Step 1: Check JOIN syntax correctness

   RIGHT JOIN requires ON clause to specify join condition. SELECT * FROM Employees RIGHT JOIN Departments ON Employees.DeptID = Departments.ID; uses correct syntax with ON clause.

2. Step 2: Identify errors in other options

   SELECT * FROM Employees LEFT JOIN Departments ON Employees.DeptID = Departments.ID; uses LEFT JOIN, not RIGHT JOIN. SELECT * FROM Employees RIGHT JOIN Departments WHERE Employees.DeptID = Departments.ID; uses WHERE instead of ON, which is incorrect for JOIN condition. SELECT * FROM Employees JOIN Departments ON Employees.DeptID = Departments.ID; uses JOIN without specifying LEFT or RIGHT, defaults to INNER JOIN.

3. Final Answer:

   SELECT * FROM Employees RIGHT JOIN Departments ON Employees.DeptID = Departments.ID; -> Option A

4. Quick Check:

   RIGHT JOIN needs ON clause [OK]

Hint: RIGHT JOIN syntax requires ON, not WHERE [OK]

Common Mistakes:

• Using WHERE instead of ON for join condition
• Confusing LEFT JOIN with RIGHT JOIN syntax
• Omitting JOIN type defaults to INNER JOIN

3. Given tables Authors and Books, what will be the result of this query?

SELECT Authors.Name, Books.Title FROM Authors LEFT JOIN Books ON Authors.ID = Books.AuthorID;

Assuming Authors has 3 rows and Books has 2 rows, with one author having no books, what will the output include?

medium

A. Only authors who have books, excluding authors without books

B. All 3 authors with their book titles; NULL for authors without books

C. All books with their authors; NULL for books without authors

D. All authors and all books, matched and unmatched

Solution

1. Step 1: Understand LEFT JOIN output

   LEFT JOIN keeps all rows from Authors (left table). For authors without matching books, Books.Title will be NULL.

2. Step 2: Analyze given data

   Authors has 3 rows, Books has 2 rows. One author has no books, so that author appears with NULL for book title.

3. Final Answer:

   All 3 authors with their book titles; NULL for authors without books -> Option B

4. Quick Check:

   LEFT JOIN keeps all left rows, fills NULLs for no match [OK]

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

Common Mistakes:

• Thinking unmatched left rows are excluded
• Confusing LEFT JOIN with INNER JOIN
• Assuming unmatched right rows appear in LEFT JOIN

4. You wrote this query but it returns fewer rows than expected:

SELECT * FROM Orders RIGHT JOIN Customers ON Orders.CustomerID = Customers.ID;

What is the likely mistake?

medium

A. Using ON instead of WHERE for join condition

B. Missing WHERE clause to filter rows

C. Not specifying table aliases

D. Using RIGHT JOIN instead of LEFT JOIN when Orders is the main table

Solution

1. Step 1: Identify which table's rows to keep

   If Orders is the main table and you want all its rows, LEFT JOIN should be used, not RIGHT JOIN.

2. Step 2: Understand RIGHT JOIN effect

   RIGHT JOIN keeps all rows from Customers (right table), so if Orders has rows without matching Customers, those rows are excluded.

3. Final Answer:

   Using RIGHT JOIN instead of LEFT JOIN when Orders is the main table -> Option D

4. Quick Check:

   Use LEFT JOIN to keep all left table rows [OK]

Hint: Use LEFT JOIN to keep all rows from first table [OK]

Common Mistakes:

• Confusing LEFT and RIGHT JOIN roles
• Expecting RIGHT JOIN to keep left table rows
• Ignoring join direction impact on row count

5. You have two tables: Students (left) and Enrollments (right). You want a list of all students, including those not enrolled in any course, showing NULL for missing enrollment data.
Which query is best and why?

Option A: SELECT * FROM Students LEFT JOIN Enrollments ON Students.ID = Enrollments.StudentID;
Option B: SELECT * FROM Enrollments RIGHT JOIN Students ON Students.ID = Enrollments.StudentID;

hard

A. Option A, because LEFT JOIN keeps all students and matches enrollments

B. Option B, because RIGHT JOIN keeps all enrollments and matches students

C. Option A, but it excludes students without enrollments

D. Option B, but it excludes students without enrollments

Solution

1. Step 1: Identify which table's rows to keep

   You want all students, even those without enrollments, so keep all rows from Students (left table).

2. Step 2: Choose JOIN type accordingly

   LEFT JOIN keeps all rows from Students and matches enrollments or shows NULL if none. RIGHT JOIN would keep all enrollments, not all students.

3. Final Answer:

   Option A, because LEFT JOIN keeps all students and matches enrollments -> Option A

4. Quick Check:

   LEFT JOIN keeps all left table rows [OK]

Hint: Keep main table first, use LEFT JOIN to keep all its rows [OK]

Common Mistakes:

• Using RIGHT JOIN but expecting all left table rows
• Switching table order without adjusting JOIN type
• Assuming RIGHT JOIN keeps left table rows