Comma JOIN

Learn the comma form of JOIN.

Writing tables separated by a comma is an implicit CROSS JOIN. Modern SQL prefers explicit CROSS JOIN.

SELECT c.name, p.name
FROM customers c, products p;

This query gives a cartesian product with old syntax.

c.name, p.name is selected from customers c, products p.

Comma JOIN — details

With a WHERE filter the comma form becomes an INNER JOIN. But readability is poor.

For example, FROM orders o, customers c WHERE o.customer_id = c.id is an older style; if you forget the WHERE it silently becomes a CROSS JOIN.

Replace comma FROM with explicit CROSS JOIN.

Show example solution

SELECT c.name, p.name
FROM customers c
CROSS JOIN products p;

Replace comma with

SELECT c.name, p.name
FROM customers c, products p
____;

Show answer

SELECT c.name, p.name
FROM customers c, products p
CROSS JOIN;