Can MySQL JOINs Work Without an ON Clause?

MySQL JOIN Without ON Condition: Understanding Cross and Inner Joins

While most database systems require an ON clause for JOIN operations, MySQL offers a unique feature that allows its omission in certain scenarios. Join and inner join in MySQL can execute without an ON condition, resulting in a cross join.

Cross Join: A Cartesian Product

A cross join, in the absence of the ON clause, generates a Cartesian product of data from multiple tables. This means every row from the first table is paired with every row from the second table, creating a significantly larger result set.

Inner Join without ON Clause

Similarly, when using an inner join without an ON clause, MySQL behaves differently from the ANSI standard. The result is the same as a cross join, effectively removing any filtering or matching criteria.

Use Cases

Cross joins can be useful in specific situations:

• When you want to create a complete list of every possible combination of rows from multiple tables.
• When you are using a database schema that does not contain foreign keys or other relationships between tables.

Recommended Practice

While MySQL allows for join queries without an ON condition, it is generally recommended to use cross join explicitly as follows:

SELECT * FROM table1 CROSS JOIN table2

This approach is more explicit and avoids confusion with inner joins.

Right Join and Left Join

Right and left outer joins require an ON clause to specify the matching condition. Therefore, the discussion regarding joins without an ON condition does not apply to these types of joins.

The above is the detailed content of Can MySQL JOINs Work Without an ON Clause?. For more information, please follow other related articles on the PHP Chinese website!