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.

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