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.

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