User's Guide
   PART 1. Working with Databases
     CHAPTER 6. Joins: Retrieving Data from Several Tables       

Natural joins

A natural join matches the rows from two tables by comparing the values from columns, one in each table, that have the same name. It restricts the results by comparing the values of columns in the two tables with the same column name. An error is reported if there is no common column name.

For example, you can join the employee and department tables using a natural join because they have only one column name in common.

SELECT emp_fname, emp_lname, dept_name
FROM employee NATURAL JOIN department
ORDER BY dept_name, emp_lname, emp_fname

emp_fname	emp_lname	dept_name
Janet	Bigelow	Finance
Kristen	Coe	Finance
James	Coleman	Finance
Jo Ann	Davidson	Finance
Denis	Higgins	Finance
Julie	Jordan	Finance
John	Letiecq	Finance
Jennifer	Litton	Finance
Mary Anne	Shea	Finance
Alex	Ahmed	Marketing
Irene	Barletta	Marketing
Barbara	Blaikie	Marketing

Column names such as description or address often cause a NATURAL JOIN to return different results than expected. Another means of specifying a join is by means of a condition, supplied within an ON phrase. Here, you have a wide number of options at your disposal.