Query to Identify every Wednesday between 2 dates

Question

Hi,
I have this code that is working well but i found it very ugly.  Any optimisation hints or simplifications?
select to_date(:startdate, 'YYYY-MM-DD') + rownum -1 as Date_
      from all_objects
     where rownum <=  to_date(:enddate, 'YYYY-MM-DD')-to_date(:startdate, 'YYYY-MM-DD')+1
having to_char(to_date(:startdate, 'YYYY-MM-DD') + rownum -1,'D')='4'   --4 = Wednesday
group by to_date(:startdate, 'YYYY-MM-DD') + rownum -1
order by to_date(:startdate, 'YYYY-MM-DD') + rownum -1
Thanks
Hugo

Answer 1

Hi, Hugo,
Here's one way:
WITH  parameters AS
    SELECT  TO_DATE (:start_date, 'YYYY-MM-DD')  AS start_date
    ,       TO_DATE (:end_date,   'YYYY-MM-DD')  AS end_date
    FROM    dual
SELECT  start_date + LEVEL - 1 AS dt
FROM    parameters
WHERE   TO_CHAR ( start_date + LEVEL - 1
                , 'fmDY'
                , 'NLS_DATE_LANGUAGE=ENGLISH'
                )   = 'WED'
CONNECT BY  LEVEL  <= 1 + end_date - start_date
The main thing is to lose the GROUP BY.  GROUP BY is for taking groups of input rows, and producing 1 one output row from them.  You're not doing anything like that in this problem.
Instead of repeating a function call multiple times in the same query (e.g., you used
to_date(:startdate, 'YYYY-MM-DD')
5 times in the query you posted) , you can do it once, in a sub-query, and use the result as many times as you need to in a super-query.
CONNECT BY from a 1-row table (or, in this case, result set) is much more efficient than getting ROWNUM from all_objects.  Also, there is a finite number of rows in all_objects.  That number is probably in the thousands, but it's still a limit.  CONNECT BY will work for millions, or more, if you need it.
I find TO_CHAR (some_date, 'DY') easier to debug and maintain than TO_CHAR (some_date, 'D').  I might get confused, and think that '3' means Wednesday, but I'm not very likely to think that 'TUE' means Wednesday, no matter how little coffee I've had.
Also, the 'D' format depends on your NLS_TERRITORY setting, which you can't control from within the query. 'DY' depends on NLS_LANGUAGE, which you can override by passing a 3rd argument to TO_CHAR.  The query above will always check for Wednesdays, even if they are called Onsdag or Mittwoch in your session, and regardless of whether it thinks that's the 3rd or the 4th day of the week.