How to use mysql JOIN without ON condition?

MySQL documentation covers this topic.

Here is a synopsis. When using join or inner join, the on condition is optional. This is different from the ANSI standard and different from almost any other database. The effect is a cross join. Similarly, you can use an on clause with cross join, which also differs from standard SQL.

A cross join creates a Cartesian product -- that is, every possible combination of 1 row from the first table and 1 row from the second. The cross join for a table with three rows ('a', 'b', and 'c') and a table with four rows (say 1, 2, 3, 4) would have 12 rows.

In practice, if you want to do a cross join, then use cross join:

from A cross join B

is much better than:

from A, B

and:

from A join B -- with no on clause

The on clause is required for a right or left outer join, so the discussion is not relevant for them.

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See some example in http://www.sitepoint.com/understanding-sql-joins-mysql-database/

You can use 'USING' instead of 'ON' as in the query

SELECT * FROM table1 LEFT JOIN table2 USING (id);

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There are several ways to do a cross join or cartesian product:

SELECT column_names FROM table1 CROSS JOIN table2;

SELECT column_names FROM table1, table2;

SELECT column_names FROM table1 JOIN table2;

Neglecting the on condition in the third case is what results in a cross join.

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