Oracle 11g Release 2

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create table itbg_order_items (oano, pcode, slno) as (
    select '0001', 'swg1', 1 from dual union all
    select '0001', 'swg2', 2 from dual union all
    select '0001', 'swg1', 3 from dual union all
    select '0001', 'swg4', 4 from dual union all
    select '0001', 'swg1', 5 from dual );
5 rows affected

create table itbg_intend_items (intend, oano, icode, slno) as (
    select 'i/1', '0001', 'swg1', 1 from dual union all
    select 'i/1', '0001', 'swg1', 3 from dual union all
    select 'i/1', '0001', 'swg2', 2 from dual union all
    select 'i/3', '0001', 'swg4', 4 from dual union all
    select 'i/3', '0001', 'swg1', 5 from dual );
5 rows affected

update itbg_intend_items set slno = null;
5 rows affected

merge into itbg_intend_items tgt
using (
  select rwd, oano, pcode, slno
    from (select oano, pcode, slno, 
                 row_number() over (partition by oano, pcode order by null) rn 
           from itbg_order_items)
    join (select rowid rwd, oano, icode pcode, 
                 row_number() over (partition by oano, icode order by null) rn 
           from itbg_intend_items)
    using (oano, pcode, rn)) src
on (tgt.icode = src.pcode and tgt.oano = src.oano and tgt.rowid = src.rwd)
when matched then update set tgt.slno= src.slno;
5 rows affected

select * from itbg_intend_items;


INTEND
OANO
ICODE
SLNO


i/1
0001
swg1
1

i/1
0001
swg1
3

i/1
0001
swg2
2

i/3
0001
swg4
4

i/3
0001
swg1
5

INTEND	OANO	ICODE	SLNO
i/1	0001	swg1	1
i/1	0001	swg1	3
i/1	0001	swg2	2
i/3	0001	swg4	4
i/3	0001	swg1	5