How to optimize the query

Question

Hi,
This below query is taking more than 1hour. So I want to optimize this query.Any suggestion is appriciated.
In Table customer_details total number of records are 40,000. Among which custno with id='J' are 7000.
select distinct(A.custno)
from
(select distinct custno,id,
case
when id='I'
then 0
else 1
end as myid from customer_details where custno not in
(select custno from customer_details where id='J'))A
group by A.custno
having sum(A.myid)>0

Answer 1

Why instead of
              from   customer_details
             where   custno not in (select   custno
                                      from   customer_details
                                     where   id = 'J')
...not simply
              from   customer_details
             where   custno != 'J'
...?