Oracle 18c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table your_table (INV_CODE, INV_NO, INV_STATUS, INV_DATE, INV_REF_CODE, INV_REF_NO) as

select 'DN', 202001, 3, date '2020-01-02', 'SO', 202002 from dual

union all

select 'DN', 202002, 1, date '2020-01-03', 'SO', 202001 from dual

union all

select 'INV', 202001, 0, date '2020-01-03', 'DN', 202002 from dual

union all

select 'INV', 202002, 2, date '2020-01-04', 'DN', 202001 from dual

union all

select 'DN', 202003, 1, date '2020-01-05', 'SO', 202005 from dual;

5 rows affected

select t2.INV_REF_CODE, t1.INV_REF_NO, t1.INV_CODE, t2.INV_NO,

t1.INV_REF_CODE as INV_REF_CODE_2, t1.INV_NO as INV_REF_NO_2

from your_table t1

join your_table t2 on t2.INV_NO = t1.INV_REF_NO

and t2.INV_CODE = 'DN'

where t1.INV_CODE = 'INV';

INV_REF_CODE	INV_REF_NO	INV_CODE	INV_NO	INV_REF_CODE_2	INV_REF_NO_2
SO	202001	INV	202001	DN	202002
SO	202002	INV	202002	DN	202001