[GENERAL] How can I group all children by their parent ?

[Arup Rakshit]

How can I group all children by their parent ? 

id email parent_id
1 t...@test.com nil
2 te...@test.com 1
3 email 1
4 email 2
5 email nil
6 email 3
 
Regards,
Arup Rakshit