Query in MySQL Father and son -

i have table have 3 levels in structure example:

1 son 2 , 2 have son 4 , 6
3 father , son 5
, 1 have other son 7
4 doesn't have son because rule of structure.

well table this:

|id_father|   |id_son|
         1              2
         2              4
         3              5
         2              6
         1              7

only want bringme query every father have 1 son, part of query:

select r.* getname r not exists (select 1 estructura r2 r.id = r2.id_son) ,  exists (select 1 estructura r2 r.id = r2.id_father ,  not exists (select 1 estructura r3 r2.id_son = r3.id_father )) 

and query :

1 , 3 want 3.

try one:

select id_father estructura group id_father having count(distinct id_son) = 1