Natural join and its risks in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a Natural Join Query

Given two tables Employees and Departments with the following data:

Employees
id | name | dept_id
1 | Alice | 10
2 | Bob | 20
3 | Carol | 10

Departments
dept_id | dept_name
10 | Sales
20 | Marketing
30 | HR

What is the output of the following SQL query?

SELECT * FROM Employees NATURAL JOIN Departments;

SQL

SELECT * FROM Employees NATURAL JOIN Departments;

A

id | name  | dept_id | dept_name
1  | Alice | 10      | Sales
3  | Carol | 10      | Sales

B

id | name  | dept_id | dept_name
1  | Alice | 10      | Sales
2  | Bob   | 20      | Marketing
3  | Carol | 10      | Sales

C

id | name  | dept_id | dept_name
2  | Bob   | 20      | Marketing
3  | Carol | 10      | Sales

D

id | name  | dept_id | dept_name
1  | Alice | 10      | Sales
2  | Bob   | 20      | Marketing

💡 Hint

Natural join matches rows based on columns with the same name in both tables.

The natural join matches rows where the dept_id is the same in both tables. All employees with matching dept_id values in Departments appear with their department names.

🧠 Conceptual

intermediate

Risk of Using Natural Join

Which of the following is a common risk when using natural join in SQL queries?

A It only works with numeric columns.

B It always requires specifying join conditions explicitly.

C It can unintentionally join tables on columns with the same name but different meanings.

D It automatically removes duplicate rows from the result.

💡 Hint

Think about what happens if two tables share column names that are not meant to be joined.

Natural join matches all columns with the same name, which can cause unexpected joins if columns share names but represent different data.

📝 Syntax

advanced

Identify the Syntax Error in Natural Join Usage

Which of the following SQL queries will cause a syntax error?

A SELECT * FROM Employees NATURAL JOIN Departments ON Employees.dept_id = Departments.dept_id;

B SELECT * FROM Employees NATURAL JOIN Departments;

C SELECT * FROM Employees NATURAL JOIN Departments USING (dept_id);

D SELECT * FROM Employees NATURAL JOIN ON Departments dept_id;

💡 Hint

Check the correct syntax for natural join and whether ON clause is allowed with it.

Natural join does not allow an ON clause. Option D uses 'NATURAL JOIN ON' which is invalid syntax.

❓ optimization

advanced

Optimizing Join Queries to Avoid Risks of Natural Join

You want to join two tables Orders and Customers on the customer ID column. To avoid risks of natural join, which query is the best optimized and safe option?

A SELECT * FROM Orders JOIN Customers ON Orders.customer_id = Customers.customer_id;

B SELECT * FROM Orders NATURAL JOIN Customers;

C SELECT * FROM Orders JOIN Customers USING (order_id);

D SELECT * FROM Orders JOIN Customers ON Orders.id = Customers.id;

💡 Hint

Explicitly specify join conditions to avoid unintended matches.

Using JOIN with ON clause specifying the exact columns to join is safer and clearer than natural join.

🔧 Debug

expert

Debugging Unexpected Results from Natural Join

You run this query:

SELECT * FROM Products NATURAL JOIN Sales;

Both tables have columns named id and date. You expected to join only on id, but the result is empty. What is the most likely cause?

A The natural join is joining on both <code>id</code> and <code>date</code>, and no rows match on both columns.

B Natural join only joins on the first column with the same name, so <code>date</code> is ignored.

C Natural join requires an explicit ON clause to work correctly.

D The tables must have the same number of columns for natural join to work.

💡 Hint

Natural join uses all columns with the same name to join rows.

Natural join matches rows where all columns with the same name have equal values. If date values differ, no rows join.