Question

Question:

How do you compare (< or >) the result of one column, against the result of another column of two select statements?

More details:

I need to get a list only of the sold out movies. To do this, I need to check if
the number on the rented_movie_count on the first select statement is equal or greater than the number on the available_movie_count on the second select statement.

First Statement

result of the 
select MOVIE_ID, count(MOVIE_ID) as rented_movies_count
from MOVIE_QUEUE
WHERE STATUS_ID=2
group by MOVIE_ID;

Second Statement

select MOVIE_ID, count(MOVIE_ID) as available_movie_count
from dvd
group by MOVIE_ID;

I am still very new to this and I am trying to learn, even suggestions about what kind of
syntax (operators, expressions, etc) should I use will help at least point me on a a directions to do more research. Thanks in advance

Answer 1

Given your database structure, a possible solution would be

SELECT rented.MOVIE_ID
      ,rented_movies_count
      ,available_movie_count
  FROM (
        SELECT MOVIE_ID, count(MOVIE_ID) AS rented_movies_count
          FROM MOVIE_QUEUE
         WHERE STATUS_ID=2
        GROUP BY MOVIE_ID
       ) rented
     ,(SELECT MOVIE_ID, count(MOVIE_ID) AS available_movie_count
         FROM dvd
       GROUP BY MOVIE_ID
      ) available
 WHERE rented.movie_id = available.movie_id
   AND rented_movies_count >= available_movie_count 
;

I’ve set up an example on http://www.sqlfiddle.com/#!4/3fc59/11 to test. But I guess that’s more or less what you’ve come up with.

But I somewhat doubt that your database structure is really optimal!? Do you really store more than one record for a singe movie_id in the table dvd?

Wouldn’t it be more useful to have exactly one record per movie_id in dvd along with some movie attributes and store the rental status of the dvds in movie_queue.status_id?

If movie_queue.status_id is defined as say

1. …available
2. …rented

Then your query would be much more easy:

SELECT MOVIE_ID
      ,count(case when status_id = 1 then 1 end) AS available_movie_count
      ,count(case when status_id = 2 then 1 end) AS rented_movies_count
  FROM MOVIE_QUEUE
 GROUP BY MOVIE_ID
HAVING count(case when status_id = 2 then 1 end) >= count(case when status_id = 1 then 1 end)
;

See for example http://www.sqlfiddle.com/#!4/13739/1