drop table t1;
create table t1 (c1 char(2) primary key, c2 char(1) not null);
insert into t1 values ('A ','A');
 
select t1.c1 from t1
 where t1.c1 in (select c2 from t1 union all select '0'||c2 c2 from t1);

select /*+ full(t1) */ t1.c1 from t1
 where t1.c1 in (select c2 from t1 union all select '0'||c2 c2 from t1);
 
1st query returns no row, 2nd query return 'A'