select sum(DAT)/count(*) from (
  select D_FLG1 as D_FLG, DAT1 as DAT from table where ...
  union all
  select D_FLG2 as D_FLG, DAT2 as DAT from table where ...
  union all
  ...
)where D_FLG...

------解决方案--------------------
楼主参考一下:
SQL> select * from flag;

     FLAG       DATA
---------- ----------
        2        100
        2        100
        2        100
        2        100
        1        100
        2        105
        2        105
        1        100
        1        100
        2        100

已选择10行。

flag=2表示有效;
flag=1表示无效.
rownum是你要取的小时数，你也可以改为区间形式。

SQL> select sum(data*decode(flag,2,1,1,0,1))/sum(decode(flag,2,1,1,0,1)) result from flag where rownum<=4;

   RESULT
----------
      100

SQL> select sum(data*decode(flag,2,1,1,0,1))/sum(decode(flag,2,1,1,0,1)) result from flag where rownum<=7;

   RESULT
----------
101.666667

SQL> select sum(data*decode(flag,2,1,1,0,1))/sum(decode(flag,2,1,1,0,1)) result from flag where rownum<=10;

   RESULT
----------
101.428571