Oracle常用函数:DateDiff() 返回两个日期之间的时间间隔自定义函数

Create Or Replace Function CDate(Datechar In Varchar2) Return Date Is  
    ReallyDo Date;  
Begin  
    Select to_date(to_char(to_date(to_char(Datechar), 'YYYY-MM-DD HH24:MI:SS'),  
                           'YYYY-MM-DD'),  
                   'YYYY-MM-DD')  
    Into ReallyDo  
    From Dual;  
    Return(ReallyDo);  
End CDate;  
  
   
  
Create Or Replace Function CDateTime(Datechar In Varchar2) Return Date Is  
    ReallyDo Date;  
Begin  
    Select to_date(to_char(to_date(to_char(Datechar), 'YYYY-MM-DD HH24:MI:SS'),  
                           'YYYY-MM-DD HH24:MI:SS'),  
                   'YYYY-MM-DD HH24:MI:SS')  
    Into ReallyDo  
    From Dual;  
    Return(ReallyDo);  
End CDateTime;  
  
   
  
Create Or Replace Function Datediff  
(  
    Datepart  In Varchar2,  
    StartDate In Varchar2,  
    EndDate   In Varchar2  
) Return Number Is  
    ReallyDo Numeric;  
Begin  
    Select Case Upper(Datepart)  
               When 'YYYY' Then  
                Trunc(Extract(Year From CDate(EndDate)) -  
                      Extract(Year From CDate(StartDate)))  
               When 'M' Then  
                Datediff('YYYY', StartDate, EndDate) * 12 +  
                (Extract(Month From CDate(EndDate)) -  
                 Extract(Month From CDate(StartDate)))  
               When 'D' Then  
                Trunc(CDate(EndDate) - CDate(StartDate))  
               When 'H' Then  
                Datediff('D', StartDate, EndDate) * 24 +  
                (to_Number(to_char(CDateTime(EndDate), 'HH24')) -  
                 to_Number(to_char(CDateTime(StartDate), 'HH24')))  
               When 'N' Then  
                Datediff('D', StartDate, EndDate) * 24 * 60 +  
                (to_Number(to_char(CDateTime(EndDate), 'MI')) -  
                 to_Number(to_char(CDateTime(StartDate), 'MI')))  
               When 'S' Then  
                Datediff('D', StartDate, EndDate) * 24 * 60 * 60 +  
                (to_Number(to_char(CDateTime(EndDate), 'SS')) -  
                 to_Number(to_char(CDateTime(StartDate), 'SS')))  
               Else  
                -29252888  
           End  
    Into ReallyDo  
    From Dual;  
    Return(ReallyDo);  
End Datediff;