Data type query

Question

SQL> select * from roomtype
  2  ;
   RT_CODE RT_DES    RT_RATE
         1 Single         70
         2 Double         80
         3 Suite         100
SQL> select * from room;
    RM_NUM    RM_TYPE
       201          1
       202          1
       203          2
       204          3
SQL> select * from reserve;
   RES_NUM   RES_ROOM RES_CHKIN RES_CHKOU RES_NAME
      1001        201 01-APR-10 03-APR-10 Dobbs
      1002        201 06-APR-10 08-APR-10 Black
      1003        201 09-APR-10 12-APR-10 Dobbs
      1004        201 16-APR-10 18-APR-10 Gregg
      1005        202 02-APR-10 04-APR-10 Jones
      1006        202 06-APR-10 08-APR-10 Giles
      1007        202 13-APR-10 14-APR-10 Mason
      1008        203 06-APR-10 11-APR-10 Ott
      1009        203 12-APR-10 14-APR-10 Smith
      1010        203 16-APR-10 18-APR-10 Jones
      1011        204 01-APR-10 06-APR-10 Wilson
   RES_NUM   RES_ROOM RES_CHKIN RES_CHKOU RES_NAME
      1012        204 09-APR-10 11-APR-10 Clay
      1013        204 11-APR-10 18-APR-10 OlsenI need help to write a query that will list rooms that would be available for a stay with check in on 4/12 and checkout on 4/16.
I was thinking about using a subquery to find the rooms that are not available then select rooms that are not in the list of unavailable rooms returned by the subquery. What do you think and how would that look that??

Answer 1

Hi,
What you suggested is a good way to do it if you need data from tables other than res_room. (For example, if you were only interested in Double rooms, or in a given price range).
Here's one way to do that:
SELECT  rm_num
FROM     room
WHERE     rm_num     NOT IN  (  SELECT  res_room
                      FROM        reserve
                  WHERE   res_checkin  < DATE '2010-04-16'
                        AND     res_checkout > DATE '2010-04-12'
                  AND        res_room     IS NOT NULL     -- See note below
               );Note: NOT IN never returns TRUE if the sub-query includes NULL. It's likely that this is impossible in your case.
If your only need data from the res_romm table (for example, just the room number) then it will usually be faster to do a query like this:
SELECT       res_room
FROM       reserve
GROUP BY  res_room
HAVING       COUNT ( CASE
                  WHEN  res_checkin  < DATE '2010-04-16'
                AND   res_checkout > DATE '2010-04-12'
                THEN  1
            END
          ) = 0
;If you'd like to post CREATE TABLE and INSERT statements for your sample data, then I could test this.
Hi, Warren,
Won't your query say the room is free if it has a reservation entirely within the target range?
For example, what if res_checkin is April 13, and res_checkout is April 14?
The interesting part of this question is: When do two ranges overlap?
It's actually much easier to aswer the converse question: When do two ranges NOT overlap?
They do not overlap if (and only if) one ends before the other begins.
Therefore, they do overlap if both end after the other begins. (The solution above assumes that res_checkin <= res_checkout, and that the target check-in date is less than or equal to the target check-out date.)