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.