SQL CROSS JOIN Operation

If you don't specify a join condition when joining two tables, database system combines each row from the first table with each row from the second table. This type of join is called a cross join or a Cartesian product. The following Venn diagram illustrates how cross join works.

The number of rows in a cross join is the product of the number of rows in each table. Here's a simple example of a cross join operation.

Example

SELECT t1.emp_id, t1.emp_name, t1.hire_date, t2.dept_name
FROM employees AS t1 CROSS JOIN departments AS t2;

After executing the above command, you get the result set something like this:

+--------+--------------+------------+------------------+
| emp_id | emp_name     | hire_date  | dept_name        |
+--------+--------------+------------+------------------+
|      1 | Ethan Hunt   | 2001-05-01 | Administration   |
|      2 | Tony Montana | 2002-07-15 | Administration   |
|      3 | Sarah Connor | 2005-10-18 | Administration   |
|      4 | Rick Deckard | 2007-01-03 | Administration   |
|      5 | Martin Blank | 2008-06-24 | Administration   |
|      1 | Ethan Hunt   | 2001-05-01 | Customer Service |
|      2 | Tony Montana | 2002-07-15 | Customer Service |
|      3 | Sarah Connor | 2005-10-18 | Customer Service |
|      4 | Rick Deckard | 2007-01-03 | Customer Service |
|      5 | Martin Blank | 2008-06-24 | Customer Service |
|      1 | Ethan Hunt   | 2001-05-01 | Finance          |
|      2 | Tony Montana | 2002-07-15 | Finance          |
|      3 | Sarah Connor | 2005-10-18 | Finance          |
|      4 | Rick Deckard | 2007-01-03 | Finance          |
|      5 | Martin Blank | 2008-06-24 | Finance          |
|      1 | Ethan Hunt   | 2001-05-01 | Human Resources  |
|      2 | Tony Montana | 2002-07-15 | Human Resources  |
|      3 | Sarah Connor | 2005-10-18 | Human Resources  |
|      4 | Rick Deckard | 2007-01-03 | Human Resources  |
|      5 | Martin Blank | 2008-06-24 | Human Resources  |
|      1 | Ethan Hunt   | 2001-05-01 | Sales            |
|      2 | Tony Montana | 2002-07-15 | Sales            |
|      3 | Sarah Connor | 2005-10-18 | Sales            |
|      4 | Rick Deckard | 2007-01-03 | Sales            |
|      5 | Martin Blank | 2008-06-24 | Sales            |
+--------+--------------+------------+------------------+

As you can see a cross join is not as useful as the other joins that we've covered in the previous chapters. Since the query didn't specify a join condition, each row from the employees table is combined with each row from the departments table. Therefore, unless you are sure that you want a Cartesian product don't use a cross join.