Sub-sub query referencing outside column

Question

The following SQL statement executes:
select
ci.name,
ci.customer_id KeyMed_Ship_to,
select cnp from
select co.customer_no_pay cnp, count(co.customer_no_pay) ccnp from ifsapp.customer_order co
where co.contract like 'OIA%'
and co.customer_no='???????'
group by co.customer_no_pay
order by count(co.customer_no_pay) desc
where rownum=1
) KeyMed_Bill_To,
select ptip.value from ifsapp.party_type_id_property ptip
where ptip.company='OIA' and ptip.party_type='Customer'
and ptip.identity=ci.customer_id and ptip.code='JDE BILL-TO'
) JDE_Bill_To,
select ptip.value from ifsapp.party_type_id_property ptip
where ptip.company='OIA' and ptip.party_type='Customer'
and ptip.identity=ci.customer_id and ptip.code='JDE SHIP-TO'
) JDE_Ship_To
from
ifsapp.customer_info ci, ifsapp.identity_invoice_info iii
where
iii.company='OIA' and iii.party_type='Customer' and iii.identity=ci.customer_id and group_id='SRG'
But really,
and co.customer_no='???????'
should be
and co.customer_no=ci.customer_id
however, it seems taht column is not "known" by the sub-subquery.
How can I use the value of the column in customer_id (ci) there?
Thanks

Answer 1

Just FYI (sorry if what I say is well-known) :
Then one is getting from subqueried columns to subqueries as tables,
there is a pitfall related to outer and inner joins.
In fact the subqueried column simulate logically an outer join like
in the example:
SQL> select deptno from dept;
    DEPTNO
        10
        20
        30
        40
        50
        60
6 rows selected.
SQL> select unique deptno from emp;
    DEPTNO
        10
        20
        30
SQL> select d.deptno, (select count(1) from emp e where e.deptno = d.deptno) cnt
  2  from dept d;
    DEPTNO        CNT
        10          3
        20          5
        30          6
        40          0
        50          0
        60          0
6 rows selected.
SQL> select d.deptno, t.cnt
  2  from dept d,
  3  (select count(1) cnt, deptno from emp e group by e.deptno) t
  4  where t.deptno = d.deptno
  5  /
    DEPTNO        CNT
        10          3
        20          5
        30          6
SQL> select d.deptno, nvl(t.cnt,0) cnt
  2  from dept d,
  3  (select count(1) cnt, deptno from emp e group by e.deptno) t
  4  where t.deptno (+) = d.deptno
  5  /
    DEPTNO        CNT
        10          3
        20          5
        30          6
        50          0
        40          0
        60          0
6 rows selected.Rgds.