How can I group all children by their parent ?

[Arup Rakshit <aruprakshit@xxxxxxxxxxxxxx>]

How can I group all children by their parent ? 

id  email         parent_id
1   test@test.com   nil
2   test1@test.com  1
3   email           1
4   email           2
5   email          nil
6   email           3

 

Regards,
Arup Rakshit