Why outer joins are needed in SQL - See It in Action

Understanding Why Outer Joins Are Needed

📖 Scenario: You work at a small bookstore. You have two tables: one with books and another with sales. Some books have sales records, but some do not because they haven't been sold yet.

🎯 Goal: Build a query that shows all books and their sales if any. If a book has no sales, it should still appear in the list with a NULL for sales. This will help the store owner see which books have not sold yet.

📋 What You'll Learn

Create a books table with columns book_id and title.

Create a sales table with columns sale_id, book_id, and quantity.

Insert exact data into both tables as specified.

Write a query using a LEFT OUTER JOIN to list all books with their sales quantities if available.

Ensure books without sales still appear with NULL in the sales columns.

💡 Why This Matters

🌍 Real World

Bookstores and many businesses need to see all items in inventory even if they have no sales yet. Outer joins help show this complete picture.

💼 Career

Understanding outer joins is essential for data analysts and database developers to write queries that handle missing or incomplete data gracefully.

1

Create the books table and insert data

Create a table called books with columns book_id (integer) and title (text). Insert these exact rows: (1, 'The Great Gatsby'), (2, '1984'), (3, 'To Kill a Mockingbird').

SQL

-- Create the books table and insert data
-- Your code here

Hint

Use CREATE TABLE to define the table and INSERT INTO to add rows.

2

Create the sales table and insert data

Create a table called sales with columns sale_id (integer), book_id (integer), and quantity (integer). Insert these exact rows: (1, 1, 5), (2, 2, 3).

SQL

CREATE TABLE books (
  book_id INTEGER,
  title TEXT
);

INSERT INTO books (book_id, title) VALUES
(1, 'The Great Gatsby'),
(2, '1984'),
(3, 'To Kill a Mockingbird');

-- Create the sales table and insert data
-- Your code here

Hint

Remember to match the column names and insert the exact rows.

3

Write a LEFT OUTER JOIN query to combine books and sales

Write a SQL query that selects books.book_id, books.title, and sales.quantity. Use a LEFT OUTER JOIN on books.book_id = sales.book_id to include all books even if they have no sales.

SQL

CREATE TABLE books (
  book_id INTEGER,
  title TEXT
);

INSERT INTO books (book_id, title) VALUES
(1, 'The Great Gatsby'),
(2, '1984'),
(3, 'To Kill a Mockingbird');

CREATE TABLE sales (
  sale_id INTEGER,
  book_id INTEGER,
  quantity INTEGER
);

INSERT INTO sales (sale_id, book_id, quantity) VALUES
(1, 1, 5),
(2, 2, 3);

-- Write the LEFT OUTER JOIN query below
-- Your code here

Hint

Use LEFT OUTER JOIN to keep all books even if no matching sales exist.

4

Complete the query with an ORDER BY clause

Add an ORDER BY books.book_id clause at the end of the query to list the books in order by their ID.

SQL

CREATE TABLE books (
  book_id INTEGER,
  title TEXT
);

INSERT INTO books (book_id, title) VALUES
(1, 'The Great Gatsby'),
(2, '1984'),
(3, 'To Kill a Mockingbird');

CREATE TABLE sales (
  sale_id INTEGER,
  book_id INTEGER,
  quantity INTEGER
);

INSERT INTO sales (sale_id, book_id, quantity) VALUES
(1, 1, 5),
(2, 2, 3);

SELECT books.book_id, books.title, sales.quantity
FROM books
LEFT OUTER JOIN sales ON books.book_id = sales.book_id
-- Add ORDER BY clause below
-- Your code here

Hint

Use ORDER BY books.book_id to sort the results by book ID.

Practice

1. Why do we need LEFT OUTER JOIN in SQL?

easy

A. To include all rows from the left table even if there is no matching row in the right table

B. To only show rows that have matching values in both tables

C. To delete rows from the left table

D. To update rows in the right table

Solution

1. Step 1: Understand the purpose of LEFT OUTER JOIN

   LEFT OUTER JOIN returns all rows from the left table and matched rows from the right table. If no match, NULLs appear for right table columns.

2. Step 2: Compare with INNER JOIN behavior

   INNER JOIN returns only rows with matching keys in both tables, excluding unmatched rows.

3. Final Answer:

   To include all rows from the left table even if there is no matching row in the right table -> Option A

4. Quick Check:

   LEFT OUTER JOIN shows all left rows [OK]

Hint: LEFT OUTER JOIN keeps all left rows, unmatched get NULLs [OK]

Common Mistakes:

• Confusing LEFT OUTER JOIN with INNER JOIN
• Thinking it deletes or updates rows
• Assuming it only shows matched rows

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

easy

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

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

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

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

Solution

1. Step 1: Recall correct LEFT OUTER JOIN syntax

   The correct syntax is: SELECT ... FROM table1 LEFT JOIN table2 ON condition; LEFT JOIN is shorthand for LEFT OUTER JOIN.

2. Step 2: Identify syntax errors in other options

   SELECT * FROM table1 JOIN LEFT OUTER table2 ON table1.id = table2.id; has JOIN LEFT OUTER which is invalid order. SELECT * FROM table1 OUTER LEFT JOIN table2 ON table1.id = table2.id; uses OUTER LEFT JOIN which is incorrect. SELECT * FROM table1 LEFT OUTER JOIN table2 WHERE table1.id = table2.id; uses WHERE instead of ON for join condition.

3. Final Answer:

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

4. Quick Check:

   LEFT JOIN ... ON ... is correct syntax [OK]

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

Common Mistakes:

• Swapping JOIN and LEFT keywords
• Using WHERE instead of ON for join condition
• Writing OUTER LEFT JOIN instead of LEFT OUTER JOIN

3. Given tables Employees and Departments where some employees have no department, what will this query return?

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

medium

A. All departments with employee names; NULL for departments without employees

B. Only employees who have a matching department

C. All employees with their department names; NULL for employees without a department

D. Only departments with employees

Solution

1. Step 1: Analyze LEFT JOIN behavior on Employees and Departments

   LEFT JOIN keeps all rows from Employees (left table). For employees without matching department, department columns show NULL.

2. Step 2: Understand output columns

   Query selects employee name and department name. Employees without department show NULL in department name.

3. Final Answer:

   All employees with their department names; NULL for employees without a department -> Option C

4. Quick Check:

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

Hint: LEFT JOIN shows all left rows, unmatched right side NULL [OK]

Common Mistakes:

• Thinking only matched rows appear
• Confusing left and right tables
• Expecting departments without employees to appear

4. What is wrong with this query if we want to list all customers and their orders, including customers with no orders?

SELECT c.name, o.order_id FROM Customers c INNER JOIN Orders o ON c.id = o.customer_id;

medium

A. INNER JOIN excludes customers without orders; should use LEFT OUTER JOIN

B. The ON clause is missing

C. The SELECT statement is missing table aliases

D. Orders table should be first in the FROM clause

Solution

1. Step 1: Understand INNER JOIN behavior

   INNER JOIN returns only rows with matching keys in both tables, so customers without orders are excluded.

2. Step 2: Identify correct join for including all customers

   LEFT OUTER JOIN keeps all customers even if no matching orders exist, showing NULL for order columns.

3. Final Answer:

   INNER JOIN excludes customers without orders; should use LEFT OUTER JOIN -> Option A

4. Quick Check:

   Use LEFT OUTER JOIN to include all left table rows [OK]

Hint: Use LEFT OUTER JOIN to include unmatched left table rows [OK]

Common Mistakes:

• Using INNER JOIN when outer join is needed
• Forgetting ON clause (though present here)
• Thinking table order in FROM matters for join type

5. You have two tables: Students and Enrollments. Some students are not enrolled in any course. Which query correctly lists all students and their courses, showing NULL for students without enrollments?

hard

A. SELECT s.name, e.course FROM Students s INNER JOIN Enrollments e ON s.id = e.student_id;

B. SELECT s.name, e.course FROM Students s RIGHT OUTER JOIN Enrollments e ON s.id = e.student_id;

C. SELECT s.name, e.course FROM Enrollments e LEFT OUTER JOIN Students s ON s.id = e.student_id;

D. SELECT s.name, e.course FROM Students s LEFT OUTER JOIN Enrollments e ON s.id = e.student_id;

Solution

1. Step 1: Identify which table has all students

   Students table contains all students, including those without enrollments.

2. Step 2: Choose join to keep all students

   LEFT OUTER JOIN with Students as left table keeps all students, adding course info or NULL if no enrollment.

3. Step 3: Evaluate other options

   INNER JOIN excludes students without enrollments. RIGHT OUTER JOIN with Enrollments left excludes students without enrollments. LEFT OUTER JOIN with Enrollments left table excludes students without enrollments.

4. Final Answer:

   SELECT s.name, e.course FROM Students s LEFT OUTER JOIN Enrollments e ON s.id = e.student_id; -> Option D

5. Quick Check:

   LEFT OUTER JOIN keeps all left table rows [OK]

Hint: LEFT OUTER JOIN with Students on left keeps all students [OK]

Common Mistakes:

• Using INNER JOIN excludes students without courses
• Swapping left and right tables in join
• Using RIGHT OUTER JOIN incorrectly