(SQL Tutorials for Citizen Data Scientist)
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
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.
Beginners Guide to SQL – SQL CROSS JOIN Operation
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