Если я правильно понял, вы хотите иметь два столбца для каждого клиента.Если это так, вот один из вариантов.
SQL> with test (cno, rno) as
2 (select 'A', 123 from dual union all
3 select 'A', 234 from dual union all
4 select 'A', 345 from dual union all
5 select 'A', 444 from dual union all
6 select 'A', 555 from dual union all
7 select 'B', 456 from dual union all
8 select 'B', 675 from dual
9 ),
10 inter as
11 (select cno, rno,
12 row_number() over (partition by cno order by cno, rno) rn,
13 round(row_number() over (partition by cno order by cno, rno) /2) grp
14 from test
15 )
16 select cno,
17 max(decode(mod(rn, 2), 1, rno)) r1,
18 max(decode(mod(rn, 2), 0, rno)) r2
19 from inter
20 group by cno, grp
21 order by cno, grp;
C R1 R2
- ---------- ----------
A 123 234
A 345 444
A 555
B 456 675
SQL>