Rule Definition
NATURAL JOIN is a type of equi-join which implicitly compares all identically-named columns of the two tables. While this a feature which may seem convenient at first, it becomes hard to maintain over time.

Remediation
Check the statement and if so, replace NATURAL JOINs with INNER JOINs.

Violation Code Sample

SELECT first_name, department_name
FROM employees
     NATURAL JOIN departments;

Fixed Code Sample

The following example:
SELECT first_name, department_name
FROM employees
     NATURAL JOIN departments;
should be rewritten as:
SELECT first_name,department_name
FROM employees
     JOIN departments
ON (employees.manager_id = departments.manager_id
     AND employees.department_id = departments.department_id);
or as:
SELECT first_name, department_name
FROM employees
     JOIN departments
     USING(manager_id, department_id);