Du kan använda reguljära uttryck. I det här fallet, regexp_like( sales_id, '^[[:digit:]]*$' )
SQL> with x as (select 12345678 code, '456' sales_id from dual union all
2 select 34534534, '678' from dual union all
3 select 23423423, 'X123' from dual)
4 select *
5 from x
6 where regexp_like( sales_id, '^[[:digit:]]*$' );
CODE SALE
---------- ----
12345678 456
34534534 678