INNER join SQL command is mostly used to join one table to it self. The biggest advantage of doing this is to get linking information from the same table. We will try to understand this with an example. The best example of INNER join will be employee table where we will keep the employee and its manager as a single record. This way by linking to the table it self we will generate a report displaying it as two linked tables. Each record will have one additional field storing the data of the manager by keeping the employee ID and we will use M_ID ( manager ID ) to link with main employee ID. This way we will link two virtual tables generated from one main table. Here is the table. You can download /copy the sql dump file to create your own MySQL table for testing.
| Main table | Managers Table | Employee table |
| id |
name |
m_id |
| 1 |
John |
2 |
| 2 |
Greek Tor |
1 |
| 3 |
Alex John |
1 |
| 4 |
Mike tour |
1 |
| 5 |
Brain J |
3 |
| 6 |
Ronald |
3 |
| 7 |
Kin |
4 |
| 8 |
Herod |
3 |
| 9 |
Alen |
2 |
| 10 |
Ronne |
1 |
|
| id |
emp_name |
| 2 |
Greek Tor |
| 1 |
John |
| 1 |
John |
| 1 |
John |
| 3 |
Alex John |
| 3 |
Alex John |
| 4 |
Mike tour |
| 3 |
Alex John |
| 2 |
Greek Tor |
| 1 |
John |
|
| id |
emp_name |
| 1 |
John |
| 2 |
Greek Tor |
| 3 |
Alex John |
| 4 |
Mike tour |
| 5 |
Brain J |
| 6 |
Ronald |
| 7 |
Kin |
| 8 |
Herod |
| 9 |
Alen |
| 10 |
Ronne |
|
In the table you can see every record has one manager id field known as m_id. We have used the unique id of the employee in the m_id field to mark who is the manager for the employee.
To generate the manager table we have used this SQL
SELECT t1.id,t1.name as emp_name from emp as t1 INNER JOIN emp as t2 on t1.id=t2.m_id
To generate the employee table we have used this SQL
SELECT t1.id,t1.name as emp_name from emp as t1 INNER JOIN emp as t2 on t2.id=t1.m_id
Now let us use inner join to create one report on who is the manager of which employee. Check this SQL
SELECT t1.id, t1.name as emp_name, t2.name as manager FROM emp as t1 INNER JOIN emp as t2 on t2.id = t1.m_id
| id |
emp_name |
manager |
| 1 |
John |
Greek Tor |
| 2 |
Greek Tor |
John |
| 3 |
Alex John |
John |
| 4 |
Mike tour |
John |
| 5 |
Brain J |
Alex John |
| 6 |
Ronald |
Alex John |
| 7 |
Kin |
Mike tour |
| 8 |
Herod |
Alex John |
| 9 |
Alen |
Greek Tor |
| 10 |
Ronne |
John |
|
|
CREATE TABLE `emp` (
`id` int(4) NOT NULL auto_increment,
`name` varchar(25) NOT NULL default '',
`m_id` int(4) NOT NULL default '0',
UNIQUE KEY `id` (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=11 DEFAULT CHARSET=latin1 AUTO_INCREMENT=11 ;
--
-- Dumping data for table `emp`
--
INSERT INTO `emp` VALUES (1, 'John', 2);
INSERT INTO `emp` VALUES (2, 'Greek Tor', 1);
INSERT INTO `emp` VALUES (3, 'Alex John', 1);
INSERT INTO `emp` VALUES (4, 'Mike tour', 1);
INSERT INTO `emp` VALUES (5, 'Brain J', 3);
INSERT INTO `emp` VALUES (6, 'Ronald', 3);
INSERT INTO `emp` VALUES (7, 'Kin', 4);
INSERT INTO `emp` VALUES (8, 'Herod', 3);
INSERT INTO `emp` VALUES (9, 'Alen', 2);
INSERT INTO `emp` VALUES (10, 'Ronne', 1);
|
