Join Operation

We can combine the columns from two or more tables by using some common column among them by using the join statement.

We have only one table in our database, let’s create one more table Departments with two columns department_id and department_name.

As we have created a new table Departments as shown in the above image. However, we haven’t yet inserted any value inside it.

Let’s insert some Departments ids and departments names so that we can map this to our Employee table.

Let’s look at the values inserted in each of the tables. Consider the following image.

Now, let’s create a python script that joins the two tables on the common column, i.e., dept_id.

Example

Output:

ID    Name    Salary    Dept_Id    Dept_Name
101   John    25000    	201    		CS
102   John    25000    	201    		CS
103   David   25000       202    		IT
104   Nick    90000   	201    		CS
105   Mike    28000   	202   		IT

Right Join

Right join shows all the columns of the right-hand side table as we have two tables in the database PythonDB, i.e., Departments and Employee. We do not have any Employee in the table who is not working for any department (Employee for which department id is null). However, to understand the concept of right join let’s create the one.

Execute the following query on the MySQL server.

This will insert an employee Alex who doesn’t work for any department (department id is null).

Now, we have an employee in the Employee table whose department id is not present in the Departments table. Let’s perform the right join on the two tables now.

Example

Output:

ID    Name    Salary    Dept_Id    Dept_Name
101      John      25000.0      201      CS
102      John      25000.0      201      CS
103      David      25000.0      202      IT
104      Nick      90000.0      201      CS
105      Mike      28000.0      202      IT
108      Alex      29900.0      None      None

Left Join

The left join covers all the data from the left-hand side table. It has just opposite effect to the right join. Consider the following example.

Example

Output:

ID    Name    Salary    Dept_Id    Dept_Name
101      John      25000.0      201      CS
102      John      25000.0      201      CS
103      David      25000.0      202      IT
104      Nick      90000.0      201      CS
105      Mike      28000.0      202      IT