Natural join and its risks in SQL - Mini Project: Build & Apply

Understanding Natural Join and Its Risks in SQL

📖 Scenario: You are working with two tables in a company database: employees and departments. Both tables have a column named department_id. You want to combine these tables to see employee names along with their department names.However, you need to understand how using a natural join works and what risks it might have when joining tables.

🎯 Goal: Build a SQL query using NATURAL JOIN to combine the employees and departments tables on their common column. Then, learn to identify potential risks of using natural joins.

📋 What You'll Learn

Create two tables: employees and departments with specified columns

Insert sample data into both tables

Write a SQL query using NATURAL JOIN to combine the tables

Explain the risk of using NATURAL JOIN when tables have multiple columns with the same name

💡 Why This Matters

🌍 Real World

Natural joins can simplify queries when tables share exactly one common column, but in real databases, tables often share multiple column names, so understanding risks helps avoid bugs.

💼 Career

Database developers and analysts must write correct join queries to combine data accurately. Knowing when to use or avoid NATURAL JOIN is important for data integrity.

1

Create the employees and departments tables

Write SQL statements to create a table called employees with columns employee_id (integer), employee_name (text), and department_id (integer). Also create a table called departments with columns department_id (integer) and department_name (text).

SQL

-- Create employees and departments tables
-- Your code here

Hint

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

2

Insert sample data into both tables

Insert these rows into employees: (1, 'Alice', 10), (2, 'Bob', 20), (3, 'Charlie', 10). Insert these rows into departments: (10, 'HR'), (20, 'Engineering').

SQL

CREATE TABLE employees (
    employee_id INTEGER,
    employee_name TEXT,
    department_id INTEGER
);

CREATE TABLE departments (
    department_id INTEGER,
    department_name TEXT
);

-- Insert data into employees and departments
-- Your code here

Hint

Use INSERT INTO statements with the exact values given.

3

Write a SQL query using NATURAL JOIN

Write a SQL query to select employee_name and department_name by joining employees and departments using NATURAL JOIN.

SQL

CREATE TABLE employees (
    employee_id INTEGER,
    employee_name TEXT,
    department_id INTEGER
);

CREATE TABLE departments (
    department_id INTEGER,
    department_name TEXT
);

INSERT INTO employees VALUES (1, 'Alice', 10);
INSERT INTO employees VALUES (2, 'Bob', 20);
INSERT INTO employees VALUES (3, 'Charlie', 10);

INSERT INTO departments VALUES (10, 'HR');
INSERT INTO departments VALUES (20, 'Engineering');

-- Write a query using NATURAL JOIN to select employee_name and department_name
-- Your code here

Hint

Use SELECT employee_name, department_name FROM employees NATURAL JOIN departments;

4

Explain the risk of using NATURAL JOIN

Add a SQL comment explaining that NATURAL JOIN can be risky because it automatically joins on all columns with the same name, which might cause unexpected results if tables have multiple columns with the same name.

SQL

CREATE TABLE employees (
    employee_id INTEGER,
    employee_name TEXT,
    department_id INTEGER
);

CREATE TABLE departments (
    department_id INTEGER,
    department_name TEXT
);

INSERT INTO employees VALUES (1, 'Alice', 10);
INSERT INTO employees VALUES (2, 'Bob', 20);
INSERT INTO employees VALUES (3, 'Charlie', 10);

INSERT INTO departments VALUES (10, 'HR');
INSERT INTO departments VALUES (20, 'Engineering');

SELECT employee_name, department_name FROM employees NATURAL JOIN departments;

-- Your comment here explaining the risk of NATURAL JOIN

Hint

Add a comment starting with -- explaining the risk of NATURAL JOIN.

Practice

1. What does a NATURAL JOIN do in SQL?

easy

A. Automatically joins tables on all columns with the same names

B. Joins tables only on columns with different names

C. Joins tables without any condition

D. Joins tables using a specified ON condition

Solution

1. Step 1: Understand the definition of NATURAL JOIN

   A NATURAL JOIN automatically matches columns with the same names in both tables and joins on those columns.

2. Step 2: Compare with other join types

   Unlike explicit ON conditions, NATURAL JOIN uses all common column names without needing to specify them.

3. Final Answer:

   Automatically joins tables on all columns with the same names -> Option A

4. Quick Check:

   NATURAL JOIN = joins on same-named columns [OK]

Hint: Natural join matches all same-named columns automatically [OK]

Common Mistakes:

• Thinking NATURAL JOIN joins on different column names
• Assuming NATURAL JOIN needs ON clause
• Believing NATURAL JOIN joins without any condition

2. Which of the following is the correct syntax for a natural join between tables Employees and Departments?

easy

A. SELECT * FROM Employees INNER JOIN Departments;

B. SELECT * FROM Employees JOIN Departments ON Employees.id = Departments.id;

C. SELECT * FROM Employees NATURAL JOIN Departments;

D. SELECT * FROM Employees CROSS JOIN Departments NATURAL;

Solution

1. Step 1: Recall the syntax of NATURAL JOIN

   The correct syntax is: SELECT columns FROM table1 NATURAL JOIN table2;

2. Step 2: Check each option

   SELECT * FROM Employees NATURAL JOIN Departments; uses the correct NATURAL JOIN syntax. SELECT * FROM Employees JOIN Departments ON Employees.id = Departments.id; uses explicit ON clause, not natural join. SELECT * FROM Employees INNER JOIN Departments; misses join condition. SELECT * FROM Employees CROSS JOIN Departments NATURAL; is invalid syntax.

3. Final Answer:

   SELECT * FROM Employees NATURAL JOIN Departments; -> Option C

4. Quick Check:

   Correct NATURAL JOIN syntax = SELECT * FROM Employees NATURAL JOIN Departments; [OK]

Hint: Natural join syntax: FROM table1 NATURAL JOIN table2 [OK]

Common Mistakes:

• Using ON clause with NATURAL JOIN
• Missing NATURAL keyword
• Placing NATURAL after JOIN keyword incorrectly

3. Given two tables:
Employees(emp_id, name, dept_id)
Departments(dept_id, dept_name, location)
What will be the result of this query?

SELECT emp_id, name, dept_name FROM Employees NATURAL JOIN Departments;

medium

A. Syntax error due to missing ON clause

B. Rows combining employees with their department names based on matching dept_id

C. Only employees with no department

D. All employees repeated for each department

Solution

1. Step 1: Identify common columns for NATURAL JOIN

   Both tables share dept_id, so NATURAL JOIN matches rows where dept_id is equal.

2. Step 2: Understand the output columns

   The query selects emp_id, name from Employees and dept_name from Departments, showing employee info with their department name.

3. Final Answer:

   Rows combining employees with their department names based on matching dept_id -> Option B

4. Quick Check:

   NATURAL JOIN matches on dept_id, returns combined rows [OK]

Hint: Natural join matches on common columns, returns combined rows [OK]

Common Mistakes:

• Expecting all employees repeated for each department
• Thinking NATURAL JOIN returns unmatched rows
• Assuming syntax error without ON clause

4. Consider these tables:
Orders(order_id, customer_id, date)
Customers(customer_id, name, date)
What is the main problem with using NATURAL JOIN on these tables?

medium

A. It will join on both customer_id and date, possibly causing incorrect matches

B. It will cause a syntax error because of duplicate column names

C. It will ignore the customer_id column and join only on date

D. It will return no rows because columns have the same name

Solution

1. Step 1: Identify columns with same names in both tables

   Both tables have customer_id and date columns.

2. Step 2: Understand NATURAL JOIN behavior

   NATURAL JOIN joins on all columns with the same names, so it will join on both customer_id and date, which may cause unintended filtering or incorrect matches.

3. Final Answer:

   It will join on both customer_id and date, possibly causing incorrect matches -> Option A

4. Quick Check:

   NATURAL JOIN joins on all same-named columns, beware unintended matches [OK]

Hint: Natural join joins on all same-named columns, watch for unintended matches [OK]

Common Mistakes:

• Thinking NATURAL JOIN causes syntax errors with duplicate columns
• Assuming it joins only on one column
• Believing it returns no rows due to same column names

5. You have two tables:
Products(product_id, name, category_id)
Categories(category_id, name)
Using NATURAL JOIN between these tables causes unexpected results. What is the best way to fix this?

hard

A. Remove the category_id column from one table

B. Use NATURAL JOIN anyway and ignore the extra matches

C. Use CROSS JOIN to avoid matching columns

D. Rename one of the name columns and use an explicit JOIN with ON clause

Solution

1. Step 1: Identify the cause of unexpected results

   Both tables have a column named name. NATURAL JOIN joins on all same-named columns, so it joins on category_id and name, causing unintended matches.

2. Step 2: Fix by renaming and using explicit join

   Renaming one name column (e.g., to category_name) and using an explicit JOIN with ON clause on category_id avoids accidental joins on name.

3. Final Answer:

   Rename one of the name columns and use an explicit JOIN with ON clause -> Option D

4. Quick Check:

   Rename columns + explicit ON join avoids NATURAL JOIN risks [OK]

Hint: Rename columns and use explicit ON join to avoid natural join risks [OK]

Common Mistakes:

• Ignoring column name conflicts with NATURAL JOIN
• Removing important columns instead of renaming
• Using CROSS JOIN which returns all combinations