NATURAL join and its risks in PostgreSQL

Introduction

NATURAL join helps combine two tables by automatically matching columns with the same names. It makes joining easier but can cause unexpected results if not used carefully.

When you want to combine two tables that share columns with the same names and you want to join on all those columns automatically.

When you have simple tables with clear matching column names and want to avoid writing explicit join conditions.

When you want a quick way to merge related data without specifying each join column.

When you trust that the tables have only the intended columns with matching names for joining.

Syntax

PostgreSQL

SELECT * FROM table1 NATURAL JOIN table2;

NATURAL JOIN automatically finds columns with the same names in both tables and joins on them.

Be careful: if tables have unexpected columns with the same name, the join may produce wrong results.

Examples

This joins employees and departments tables using all columns with the same names, like department_id.

PostgreSQL

SELECT * FROM employees NATURAL JOIN departments;

Joins employees and salaries on all common columns, then selects name and salary.

PostgreSQL

SELECT name, salary FROM employees NATURAL JOIN salaries;

Sample Program

This example creates two tables with a common column department_id. The NATURAL JOIN combines rows where department_id matches, showing employee info with their department name.

PostgreSQL

CREATE TABLE employees (
  employee_id INT,
  name TEXT,
  department_id INT
);

CREATE TABLE departments (
  department_id INT,
  department_name TEXT
);

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

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

SELECT * FROM employees NATURAL JOIN departments;

Important Notes

If tables have multiple columns with the same name, NATURAL JOIN uses all of them to match rows, which might be more than you want.

Adding or changing columns in tables later can break NATURAL JOIN queries silently if new columns share names unexpectedly.

It is safer to use explicit JOIN ... ON ... conditions when you want full control over which columns to join.

Summary

NATURAL JOIN automatically joins tables on all columns with the same names.

It is easy to write but can cause unexpected results if tables have unintended matching columns.

Use NATURAL JOIN only when you are sure about the column names and want a quick join without specifying conditions.

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