NATURAL join and its risks in PostgreSQL - Mini Project: Build & Apply

Understanding NATURAL JOIN and Its Risks in PostgreSQL

📖 Scenario: You are working with two tables in a PostgreSQL database for a small bookstore. One table stores authors and the other stores books. Both tables have a column named author_id. You want to combine data from these tables using a NATURAL JOIN to see which books belong to which authors.However, you also want to understand the risks of using NATURAL JOIN when tables have multiple columns with the same name.

🎯 Goal: Build SQL queries step-by-step to perform a NATURAL JOIN between the authors and books tables. Then, add a second column with the same name in both tables to see how NATURAL JOIN behaves and why it can be risky.

📋 What You'll Learn

Create two tables: authors and books with specified columns

Insert exact sample data into both tables

Write a NATURAL JOIN query to combine the tables

Add a second common column to both tables and observe the effect on NATURAL JOIN

💡 Why This Matters

🌍 Real World

In real databases, tables often share multiple column names. Understanding NATURAL JOIN helps avoid bugs when combining data.

💼 Career

Database developers and analysts must write safe and clear join queries to ensure correct data results and avoid unexpected data loss or duplication.

1

Create tables and insert initial data

Create a table called authors with columns author_id (integer) and author_name (text). Create a table called books with columns book_id (integer), title (text), and author_id (integer). Insert these exact rows into authors: (1, 'Alice Walker'), (2, 'Mark Twain'). Insert these exact rows into books: (101, 'The Color Purple', 1), (102, 'Adventures of Huckleberry Finn', 2).

PostgreSQL

-- Create authors table
-- Create books table
-- Insert data into authors
-- Insert data into books

Hint

Use CREATE TABLE statements for both tables. Then use INSERT INTO with the exact values given.

2

Write a NATURAL JOIN query

Write a SQL query that uses NATURAL JOIN to combine authors and books tables. Select all columns from the result. Use the exact table names authors and books.

PostgreSQL

SELECT * FROM authors
-- Add NATURAL JOIN with books below

Hint

Use SELECT * FROM authors NATURAL JOIN books; to join on the common column author_id.

3

Add a second common column to both tables

Alter the authors table to add a new column called country (text). Alter the books table to add a new column called country (text). Update the authors table to set country to 'USA' for all rows. Update the books table to set country to 'USA' for all rows.

PostgreSQL

ALTER TABLE authors ADD COLUMN country TEXT;
-- Add country column to books and update values below

Hint

Use ALTER TABLE to add the country column to both tables. Then use UPDATE to set the value to 'USA'.

4

Run NATURAL JOIN again and observe the risk

Write a SQL query that uses NATURAL JOIN to join authors and books again after adding the country column. Select all columns from the result. Use the exact table names authors and books. This will show how NATURAL JOIN joins on both author_id and country columns, which can cause unexpected results.

PostgreSQL

SELECT * FROM authors NATURAL JOIN books; -- Observe the join on both author_id and country

Hint

Run the same NATURAL JOIN query again after adding the country column to see how it affects the join.

Practice

1. What does a NATURAL JOIN do in PostgreSQL?

easy

A. It joins tables automatically on all columns with the same names.

B. It joins tables only on the primary key columns.

C. It joins tables using a user-specified condition.

D. It joins tables without any condition, creating a Cartesian product.

Solution

1. Step 1: Understand NATURAL JOIN behavior

   NATURAL JOIN matches columns with the same names in both tables automatically.

2. Step 2: Compare with other join types

   Unlike explicit JOIN ON, NATURAL JOIN does not require specifying join conditions; it uses all common column names.

3. Final Answer:

   It joins tables automatically on all columns with the same names. -> Option A

4. Quick Check:

   NATURAL JOIN = automatic matching columns [OK]

Hint: NATURAL JOIN matches all same-named columns automatically [OK]

Common Mistakes:

• Thinking NATURAL JOIN uses only primary keys
• Confusing NATURAL JOIN with CROSS JOIN
• Assuming NATURAL JOIN requires explicit ON condition

2. Which of the following is the correct syntax for a NATURAL JOIN between tables employees and departments?

easy

A. SELECT * FROM employees NATURAL JOIN departments;

B. SELECT * FROM employees JOIN departments ON employees.id = departments.id;

C. SELECT * FROM employees CROSS JOIN departments;

D. SELECT * FROM employees INNER JOIN departments USING (id);

Solution

1. Step 1: Identify NATURAL JOIN syntax

   The correct syntax uses the keyword NATURAL JOIN directly between table names.

2. Step 2: Compare options

   SELECT * FROM employees NATURAL JOIN departments; uses NATURAL JOIN correctly; others use different join types or explicit conditions.

3. Final Answer:

   SELECT * FROM employees NATURAL JOIN departments; -> Option A

4. Quick Check:

   Correct NATURAL JOIN syntax = SELECT * FROM employees NATURAL JOIN departments; [OK]

Hint: NATURAL JOIN keyword goes directly between table names [OK]

Common Mistakes:

• Using ON clause with NATURAL JOIN
• Confusing NATURAL JOIN with USING clause
• Writing CROSS JOIN instead of NATURAL JOIN

3. Given two tables:
students(id, name, class)
enrollments(id, class, grade)
What will be the result of this query?

SELECT * FROM students NATURAL JOIN enrollments;

medium

A. All rows from students combined with all rows from enrollments.

B. Rows where both id and class match in both tables.

C. Rows where only class matches in both tables.

D. Rows where only id matches in both tables.

Solution

1. Step 1: Identify common columns

   Both tables share columns id and class.

2. Step 2: Understand NATURAL JOIN behavior

   NATURAL JOIN matches on all columns with the same names, so it joins on both id and class.

3. Final Answer:

   Rows where both id and class match in both tables. -> Option B

4. Quick Check:

   NATURAL JOIN matches all common columns [OK]

Hint: NATURAL JOIN uses all same-named columns to join [OK]

Common Mistakes:

• Assuming join on only one common column
• Confusing NATURAL JOIN with JOIN USING
• Expecting Cartesian product instead of filtered rows

4. Consider these tables:
products(id, name, price)
sales(id, product_id, quantity)
What is the problem with this query?

SELECT * FROM products NATURAL JOIN sales;

medium

A. It will return all rows without filtering.

B. It will cause a syntax error because NATURAL JOIN requires USING clause.

C. It will join on id columns, which are different keys, causing incorrect results.

D. It will join correctly on product_id automatically.

Solution

1. Step 1: Identify common columns

   Both tables have a column named id, but they represent different things.

2. Step 2: Understand NATURAL JOIN risk

   NATURAL JOIN joins on all same-named columns, so it joins on id which mismatches keys, causing wrong results.

3. Final Answer:

   It will join on id columns, which are different keys, causing incorrect results. -> Option C

4. Quick Check:

   Unintended column match = wrong join [OK]

Hint: Check if same-named columns mean same data before NATURAL JOIN [OK]

Common Mistakes:

• Assuming NATURAL JOIN always joins correctly
• Expecting NATURAL JOIN to use foreign keys automatically
• Ignoring column name conflicts causing wrong joins

5. You have two tables:
orders(order_id, customer_id, date)
customers(customer_id, name, city)
You want to join them but avoid errors from unexpected columns with the same name. Which is the safest approach?

hard

A. Use NATURAL JOIN only if you are sure about column names and their meaning.

B. Always use NATURAL JOIN for simplicity, ignoring column details.

C. Use CROSS JOIN and filter results manually.

D. Use INNER JOIN with explicit ON condition specifying join columns.

Solution

1. Step 1: Understand risks of NATURAL JOIN

   NATURAL JOIN can cause errors if tables have unintended same-named columns.

2. Step 2: Choose safer join method

   INNER JOIN with explicit ON condition avoids ambiguity by specifying exact join columns.

3. Final Answer:

   Use INNER JOIN with explicit ON condition specifying join columns. -> Option D

4. Quick Check:

   Explicit ON join = safest to avoid NATURAL JOIN risks [OK]

Hint: Use explicit ON join to avoid NATURAL JOIN surprises [OK]

Common Mistakes:

• Blindly using NATURAL JOIN without checking columns
• Using CROSS JOIN expecting filtered results
• Ignoring join conditions leads to wrong data