sql - Explanation of self-joins

Question

I don't understand the need for self-joins. Can someone please explain them to me?

A simple example would be very helpful.

Answer 1

You can view self-join as two identical tables. But in normalization, you cannot create two copies of the table so you just simulate having two tables with self-join.

Suppose you have two tables:

Table emp1

Id Name Boss_id            
1   ABC   3                   
2   DEF   1                   
3   XYZ   2                   

Table emp2

Id Name Boss_id            
1   ABC   3                   
2   DEF   1                   
3   XYZ   2                   

Now, if you want to get the name of each employee with his or her boss' names:

select c1.Name , c2.Name As Boss
from emp1 c1
    inner join emp2 c2 on c1.Boss_id = c2.Id

Which will output the following table:

Name  Boss
ABC   XYZ
DEF   ABC
XYZ   DEF