Oracle 10g的数据类型转换问题

数据库版本 oracle 10.2.0.1

第一条sql
drop table v_11;
create table v_11 as
select sum(b.amount*rate/100) amount

                     from pay_due_t b,document_t a,v_t c
              where   a.serial_number=b.serial_number
              and b.currency_code=c.currency_code
              and c.year=(to_char(deal_date,'yyyy'))
              and c.month=to_number(to_char(deal_date,'mm'))
 and deal_date  between trunc(to_date('07-09-2012','mm-dd-yyyy'),'yyyy') and to_date('07-09-2012 23:59:59','mm-dd-yyyy hh24:mi:ss')
                            ;
              select sum(amount) from v_11;
第二条
select sum(b.amount*rate/100) amount

                     from pay_due_t b,document_t a,v_t c
              where   a.serial_number=b.serial_number
              and b.currency_code=c.currency_code
              and year=(to_char(deal_date,'yyyy'))
              and month=to_number(to_char(deal_date,'mm'))
 and deal_date  between trunc(to_date('07-09-2012','mm-dd-yyyy'),'yyyy') and to_date('07-09-2012 23:59:59','mm-dd-yyyy hh24:mi:ss');

 

其中 v_t.year v_t.month 都是varchar2类型
两者看起来运行结果是一样的，实际却不一样的
但因为数据类型的不同 to_number的使用导致了错误结果
and c.month=to_number(to_char(deal_date,'mm'))
修改成               

and c.month=to_char(to_number(to_char(deal_date,'mm')))
结果就一致了