NATURAL join and its risks in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of NATURAL JOIN with overlapping columns

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

What is the result of the query:

SELECT * FROM Employees NATURAL JOIN Departments;

PostgreSQL

SELECT * FROM Employees NATURAL JOIN Departments;

A

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

B

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

C

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

D

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

💡 Hint

NATURAL JOIN matches rows based on all columns with the same name in both tables.

NATURAL JOIN automatically joins on columns with the same name, here 'dept_id'. All employees with matching dept_id get joined with their department name.

🧠 Conceptual

intermediate

Risk of unintended column matches in NATURAL JOIN

Why can using NATURAL JOIN be risky in queries involving multiple tables?

A Because NATURAL JOIN joins on all columns with the same name, it may join on unintended columns causing incorrect results.

B Because NATURAL JOIN only works with numeric columns, it cannot join text columns.

C Because NATURAL JOIN requires explicit ON conditions, it is verbose and error-prone.

D Because NATURAL JOIN always performs a cross join, it returns too many rows.

💡 Hint

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

NATURAL JOIN automatically joins on all columns with the same name. If tables share columns that are not intended as keys, the join may produce wrong or unexpected results.

📝 Syntax

advanced

Identify the error in NATURAL JOIN usage

Which of the following queries will cause a syntax error in PostgreSQL?

A SELECT * FROM Employees NATURAL JOIN Departments;

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

C SELECT * FROM Employees NATURAL JOIN;

D SELECT * FROM Employees INNER JOIN Departments ON Employees.dept_id = Departments.dept_id;

💡 Hint

Check if the NATURAL JOIN syntax is complete and correct.

Option C is missing the table name after NATURAL JOIN, causing a syntax error. NATURAL JOIN does not accept an ON clause, so option B is invalid syntax as well.

❓ query_result

advanced

Effect of adding a new column with same name on NATURAL JOIN

Consider two tables:

Orders
order_id | customer_id | status
1 | 101 | shipped
2 | 102 | pending

Shipments
shipment_id | order_id | status
501 | 1 | delivered
502 | 2 | in transit

What is the result of:

SELECT * FROM Orders NATURAL JOIN Shipments;

PostgreSQL

SELECT * FROM Orders NATURAL JOIN Shipments;

A

order_id | customer_id | status | shipment_id
1        | 101         | shipped| 501
2        | 102         | pending| 502
NULL     | NULL        | NULL   | 502

B

order_id | customer_id | status | shipment_id
1        | 101         | shipped| 501
2        | 102         | pending| 502

C

order_id | customer_id | status | shipment_id
1        | 101         | shipped| 501

D

order_id | customer_id | status | shipment_id
(No rows returned)

💡 Hint

NATURAL JOIN joins on all columns with the same name, including 'status'.

Since both tables have 'order_id' and 'status' columns, NATURAL JOIN joins on both. Only rows where both 'order_id' and 'status' match appear. Here, no rows match on both columns, so the join returns no rows.

🔧 Debug

expert

Diagnose unexpected results from NATURAL JOIN

A developer runs this query:

SELECT * FROM Products NATURAL JOIN Inventory;

Both tables have columns: product_id, name, and quantity.
They expect to get all products with their inventory quantities, but the result is empty.
What is the most likely cause?

A The NATURAL JOIN is joining on all columns with the same name, including 'name' and 'quantity', so only rows matching all three columns appear, causing empty result.

B The query is missing an ON clause specifying the join condition.

C The tables have no common columns, so NATURAL JOIN returns empty.

D The query should use LEFT JOIN instead of NATURAL JOIN to get all products.

💡 Hint

Think about how NATURAL JOIN decides which columns to join on.

NATURAL JOIN uses all columns with the same name as join keys. If 'name' and 'quantity' differ between tables, no rows match on all columns, resulting in empty output.