把日期 格式 09-7月-04 转换成2004-09-07 23:00 出现文字与格式字符串不匹配？

Question

t2ashipmentno_KPI.Act_Atamit 字段为date型          存放   2004-09-17 12:00
t2ashipmentno_kpi.act_insertdate 字段为varchar2型  存放   16-9月-04

Answer 1

select (t2ashipmentno_KPI.Act_Atamit - to_date(t2ashipmentno_kpi.act_insertdate,'dd-mon-yy')||' 23:00') * 24 from t2ashipmentno_kpi where act_shipfrom_province='江苏';

Answer 2

declare
    d1 date;
    begin
    d1:=to_date(sysdate,'yyyy-mm-dd');
    dbms_output.put_line(d1);
    end;
文字与字符串格式不符

Answer 3

select (t2ashipmentno_KPI.Act_Atamit - to_date(t2ashipmentno_kpi.act_insertdate)||' 23:00','dd-mon-yy hh24:mi') * 24 from t2ashipmentno_kpi where act_shipfrom_province='江苏';

Answer 4

declare
    d1 date;
    begin
    d1:=to_char(sysdate,'yyyy-mm-dd');
    dbms_output.put_line(d1);
    end;

Answer 5

declare
    d1 varchar2(20);
    begin
    d1:=to_char(sysdate,'yyyy-mm-dd');
    dbms_output.put_line(d1);
    end;sysdate本来就是日期型，为什么要转换，只有将d1定为字符型，才要转换啊

Answer 6

select trunc((t2ashipmentno_KPI.Act_Atamit - to_date(t2ashipmentno_kpi.act_insertdate||' 23:00','dd-mon-yy hh24:mi')) * 24,0) as weekly from t2ashipmentno_kpi where act_shipfrom_province='江苏';