select sum(c) from tbl where A='m';

------解决方案--------------------
select * from tb1 where c in(select c from tb1 group by c having count(1)>1);

------解决方案--------------------
探讨


select * from tb1 where c in(select c from tb1 group by c having count(1)>1);


------解决方案--------------------
SQL code
create table tb1
(
  a char(1),
  b char(3),
  c int
);
insert tb1 select
'm', '3-1', 34 union select
'f', '3-1', 29 union select
'm', '3-2', 33 union select
'f', '3-2', 30 union select
'm', '3-3', 33;


create table tbss
(
  b char(3),
  count_boy int
);

insert tbss
select b,sum(c) as count_boy
from tb1
where a='m'
group by b;

select s.b
from tbss s join(
SELECT count_boy
from tbss
group by count_boy
having count(*)>1) z on s.count_boy=z.count_boy
/*
3-2
3-3
*/