Problem to select not in I want to display the values that are between 0 and 10 and dont exist in the current table

For example :


Field
1
2
3
5
9
8


Result expected:


7
4 


The problem is that the column field is a varchar, so i have first convert to number and then make the validations

SELECT TO_NUMBER(yy.ent_sub_dealer_code,'9') FROM a_entidades yy 
where TO_NUMBER(yy.ent_sub_dealer_code,'9') not in (SELECT LEVEL n
                                                   FROM DUAL
                                                   CONNECT BY LEVEL &lt;= 9     )


Any suggestions?
create table t1 (x varchar2(2));

insert into t1 (x) values ('1');
insert into t1 (x) values ('2');
insert into t1 (x) values ('3');
insert into t1 (x) values ('5');
insert into t1 (x) values ('6');
insert into t1 (x) values ('8');
insert into t1 (x) values ('9');
insert into t1 (x) values ('10');
insert into t1 (x) values ('11');
commit;

SQL&gt;  select level as x
  2     from dual
  3  connect by level &lt;= 9
  4    minus
  5   select to_number(x)
  6     from t1;

         X
----------
         4
         7

2 rows selected.