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.

snippet

create table Departments (Dept_id int(20) primary key not null, Dept_Name varchar(20) not null);

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.

snippet

insert into Departments values (201, "CS");
insert into Departments values (202, "IT");

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

snippet

import mysql.connector

#Create the connection object 
myconn = mysql.connector.connect(host = "localhost", user = "root",passwd = "google",database = "PythonDB")

#creating the cursor object
cur = myconn.cursor()

try:
    #joining the two tables on departments_id
    cur.execute("select Employee.id, Employee.name, Employee.salary, Departments.Dept_id, Departments.Dept_Name from Departments join Employee on Departments.Dept_id = Employee.Dept_id")
    print("ID    Name    Salary    Dept_Id    Dept_Name")
    for row in cur:
        print("%d    %s    %d    %d    %s"%(row[0], row[1],row[2],row[3],row[4]))
        
except:
    myconn.rollback()

myconn.close()

Output:

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.

snippet

insert into Employee(name, id, salary, branch_name) values ("Alex",108,29900,"Mumbai");

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

snippet

import mysql.connector

#Create the connection object 
myconn = mysql.connector.connect(host = "localhost", user = "root",passwd = "google",database = "PythonDB")

#creating the cursor object
cur = myconn.cursor()

try:
    #joining the two tables on departments_id
    result = cur.execute("select Employee.id, Employee.name, Employee.salary, Departments.Dept_id, Departments.Dept_Name from Departments right join Employee on Departments.Dept_id = Employee.Dept_id")
    
	print("ID    Name    Salary    Dept_Id    Dept_Name")
    
	for row in cur:
        print(row[0],"    ", row[1],"    ",row[2],"    ",row[3],"    ",row[4])
    
    
        
except:
    myconn.rollback()

myconn.close()

Output:

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

snippet

import mysql.connector

#Create the connection object 
myconn = mysql.connector.connect(host = "localhost", user = "root",passwd = "google",database = "PythonDB")

#creating the cursor object
cur = myconn.cursor()

try:
    #joining the two tables on departments_id
    result = cur.execute("select Employee.id, Employee.name, Employee.salary, Departments.Dept_id, Departments.Dept_Name from Departments left join Employee on Departments.Dept_id = Employee.Dept_id")
    print("ID    Name    Salary    Dept_Id    Dept_Name")
    for row in cur:
        print(row[0],"    ", row[1],"    ",row[2],"    ",row[3],"    ",row[4])
    
    
        
except:
    myconn.rollback()

myconn.close()

Output:

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