Search notes:

ORA-01790: expression must have same datatype as corresponding expression

Cause ORA-01790:

select    1     as  x  from dual union all
select   'two'  as  x  from dual;

Cause another ORA-01790:

select   'two'  as  x  from dual union all
select    2     as  x  from dual;

Use to_char to prevent the error:

select   to_char(1) as  x  from dual union all
select   'two'      as  x  from dual;

See also