Equijoins and Natural Joins

An equijoin is a join in which the values in the columns being joined are compared for equality, and all columns in the tables being joined are included in the results.

The following query is an example of an equijoin:

SELECT *
FROM authors, publishers
WHERE authors.city = publishers.city

In the results of this statement, the city column appears twice. Since there's usually no point in repeating the same information, one of these two identical columns can be eliminated by restating the query. The result is called a natural join. You can restate the preceding query to form a natural join, as follows:

SELECT publishers.pub_id, publishers.pub_name,
    publishers.state, authors.*
FROM publishers, authors
WHERE publishers.city = authors.city

In this example, publishers.city does not appear in the results.