SQL CROSS JOIN Operation
In this tutorial you will learn how to fetch data from two tables using SQL cross join.
Using Cross Joins
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.
To understand this easily, let's look at the following employees and departments tables.
+--------+--------------+------------+---------+ | emp_id | emp_name | hire_date | dept_id | +--------+--------------+------------+---------+ | 1 | Ethan Hunt | 2001-05-01 | 4 | | 2 | Tony Montana | 2002-07-15 | 1 | | 3 | Sarah Connor | 2005-10-18 | 5 | | 4 | Rick Deckard | 2007-01-03 | 3 | | 5 | Martin Blank | 2008-06-24 | NULL | +--------+--------------+------------+---------+ |
+---------+------------------+ | dept_id | dept_name | +---------+------------------+ | 1 | Administration | | 2 | Customer Service | | 3 | Finance | | 4 | Human Resources | | 5 | Sales | +---------+------------------+ |
|
Table: employees |
Table: departments |
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
Try this code »SELECT t1.emp_id, t1.emp_name, t1.hire_date, t2.dept_name
FROM employees AS t1 CROSS JOIN departments AS t2;Tip: A cross join creates a Cartesian product or multiplication of all rows in one table with all rows in another. So, for example, if one table has 5 rows and another has 10 rows, a cross-join query produces 50 rows, the product of 5 and 10.
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.
