SQL Joins

Consider two tables employee and salary.

Employee

Employee_id	Name	Age
1	George	56
2	Tom	23
3	Lee	35
4	Jade	49

Salary

Salary_id	Employee_id	Amount
1	4	45000
2	5	50500
3	1	17500
4	2	24000
5	6	17000

Inner Join

select ep.name, sy.amount
from employee ep
inner join salary sy on sy.employee_id = ep.employee_id

The above code will return rows with employee_id in both the tables.

Name	Amount
Jade	45000
George	17500
Tom	24000

Left Join

select ep.name, sy.amount
from employee ep
left join salary sy on sy.employee_id = ep.employee_id

The above code will return all the rows from the employee table.

If no matching employee_id from the employee table is found in the salary table, it returns a null value.

Name	Amount
George	17500
Tom	24000
Lee	NULL
Jade	45000

Right Join

select ep.name, sy.amount
from employee ep
right join salary sy on sy.employee_id = ep.employee_id

The above code will return all the rows from the salary table.

If no matching employee_id from the salary table is found in the employee table a null value is returned.

Name	Amount
Jade	45000
NULL	50500
George	17500
Tom	24000
NULL	17000

NOTE

select ep.name, sy.amount
from employee ep
left join salary sy on sy.employee_id = ep.employee_id

is the same as

select ep.name, sy.amount
from salary sy
right join employee ep on sy.employee_id = ep.employee_id