每日考勤統計

最近一直在做公司的每日考勤這一塊，用的是mysql 在寫的時候遇到了不少的麻煩其一在選取有效工作日時（attendance_time 存的是考勤資料，_atte_holidays 這裡面存的是節假日的資料），我用了一條sql語句來查詢

select distinct(date_format(tt.attendance_time,'%y-%m-%d')) from t_attendance_time tt

where date_format(tt.attendance_time,'%y-%m-%d') between '2011-02-04' and '2011-03-04' and

tt.attendance_time not in(

select distinct(t.attendance_time) from t_attendance_time t,t_atte_holidays th where

date_format(t.attendance_time,'%y-%m-%d') between date_format(th.date_start,'%y-%m-%d') and date_format(th.date_end,'%y-%m-%d') and

date_format(t.attendance_time,'%w') not in ('0','6') )

and date_format(tt.attendance_time,'%w') not in ('0','6') order by tt.attendance_time desc

結果是出來了，可這條語句執行起來超慢，受不了。經過我不斷思考把兩個表並起來查：又寫成了這樣：

select distinct(date_format(tt.attendance_time,'%y-%m-%d')) from t_attendance_time tt,t_atte_holidays th

where date_format(tt.attendance_time,'%y-%m-%d') between '2011-02-04' and '2011-03-04' and

date_format(tt.attendance_time,'%y-%m-%d') not between date_format(th.date_start,'%y-%m-%d') and date_format(th.date_end,'%y-%m-%d')

and date_format(tt.attendance_time,'%w') not in ('0','6')

order by tt.attendance_time desc

我想過了，兩者的查詢結果是一樣的。後者查詢速度是超快的。但對為啥會變快了還是不太了解有哪位對資料庫熟悉請你不吝賜教