MySQL CROSS JOIN 6

MySQL Cross Join

MySQL Cross Join returns the Cartesian product of both the tables. The Cross Join in MySQL does not require any common column to join two tables. The Cartesian product means Number of Rows present in Table 1 Multiplied by Number of Rows present in Table 2.

SQL Cross Join Syntax

The basic syntax of the Cross Join in MySQL is

SELECT Table1.Column(s), Table2.Column(s),
FROM Table1
 CROSS JOIN
     Table2 

--OR We can Simply Write it as
SELECT Table1. Column(s), Table2. Column(s),
FROM Table1, Table2

Let us see the visual representation of the MySQL Cross join for better understanding.

MySQL CROSS JOIN 1

From the above screenshot, you can easily understand that the MySQL Cross join displays the Cartesian product of two tables. It means, every record in Table A is combining with every record Table B

For this MySQL Cross Join example, we are going to use Employ, and Department tables present in our company Database. Data present in employ is:

Left Table rows 10

Data present in the MySQL Department is:

right table rows 11

MySQL Cross Join Examples

The following are the list of ways that we can use this to combine two or get information (records) from two or more tables.

MySQL Cross Join using Select *

The following Query display all the columns present in employ, and Department tables

USE company;
SELECT * FROM employ
   CROSS JOIN department;

If you observe the below screenshot, It is displaying 135 records. It means 15 rows from the Employ multiplies by 9 rows in the Department table

MySQL CROSS JOIN 0

NOTE: The DeptID column repeated twice, which might be annoying to the end-user. To avoid unwanted columns, I suggest you select individual column names. Please avoid SELECT * Statement in it.

Select Few Columns

As we said before, please place the required columns after the SELECT Statement to avoid unwanted columns

USE company;
SELECT First_Name, Last_Name, Education, 
       DepartmentName, Standard_Salary,
       Yearly_Income, Sales
FROM employ
	CROSS JOIN department;
MySQL CROSS JOIN 2

Ambiguous Columns in MySQL Cross Join

The above query works perfectly as long as the column names from both tables (employ, and Department) are different. What happens if they had the same column names in both the tables? Well, with the above-specified approach, you end up in a mess. Let us see how to resolve the issue.

Let me show you one practical example of MySQL cross Join ambiguous column. As you can see, we are using the same query. But, we added DepID from the department table as an additional column.

USE company;
SELECT First_Name, Last_Name, Education, 
	DeptID, DepartmentName, Standard_Salary,
        Yearly_Income, Sales
FROM employ
	CROSS JOIN department;

As you can see from the screenshot below, it is throwing an error: Ambiguous column DeptID. It is because the DeptID column is present in both the tables, and MySQL query doesn’t know which column you are asking it to display.

MySQL CROSS JOIN 3

To resolve this kind of issue, you regularly have to use the table name before the column name. The following query is using the ALIAS table name before the column names. By this approach, we can inform the query that we are looking for DepID belonging to the department table.

We can simply write the above query as:

USE company;
SELECT  emp.First_Name,	emp.Last_Name, emp.Education, 
	    dept.DeptID,dept.DepartmentName, dept.Standard_Salary,
        emp.Yearly_Income, emp.Sales
FROM employ AS emp
	CROSS JOIN department AS dept;
MySQL CROSS JOIN 4

NOTE: It is always best practice to use the table name before the Column name in it. For example, SELECT employ.First_Name

MySQL Cross Join Where Clause Example

In this example, we show you, How to use Where clause (to apply filter) along with this. I suggest you refer Where Clause article.

USE company;
SELECT  emp.First_Name,	emp.Last_Name, emp.Education, 
	dept.DepartmentName, dept.Standard_Salary,
        emp.Yearly_Income, emp.Sales
FROM employ AS emp
	CROSS JOIN department AS dept
WHERE Yearly_Income > 1000000;
MySQL CROSS JOIN 5

Command prompt Example

Let me show you how to write this query using the command prompt. In this example, we show you how to use Order By clause along with it. I suggest you refer to Order By article.

USE company;
SELECT  emp.First_Name, 
	emp.Last_Name, 
        emp.Education, 
	dept.DepartmentName, dept.Standard_Salary,
        emp.Yearly_Income, emp.Sales
FROM employ AS emp
	CROSS JOIN department AS dept
ORDER BY First_Name, Last_Name;
MySQL CROSS JOIN 6