MySQL Sum multiple column values with conditions

Question

I have the following schema (two tables):

**APPS**

   | ID (bigint)  | USERID (Bigint) | USAGE_START_TIME (datetime)    | 
    ------------------------------------------------------------------
   |  1           |        12       |         2013-05-03 04:42:55    |
   |  2           |        12       |         2013-05-12 06:22:45    |
   |  3           |        12       |         2013-06-12 08:44:24    |
   |  4           |        12       |         2013-06-24 04:20:56    |
   |  5           |        13       |         2013-06-26 08:20:26    |
   |  6           |        13       |         2013-09-12 05:48:27    |


**USAGE** 

   | ID (bigint)  | APPID (bigint) |   DEVICEID (bigint)  | HIGH_COUNT (bigint) |  MEDIUM_COUNT (bigint)  |
    --------------------------------------------------------------------------------------------------------
   |  1           |        1       |                  2    |       400           |                   200   |
   |  2           |        1       |                  3    |       200           |                   100   |
   |  3           |        2       |                  3    |       350           |                    40   |
   |  4           |        3       |                  4    |         2           |                   400   |
   |  5           |        4       |                  2    |         4           |                    30   |
   |  6           |        5       |                  3    |        50           |                   300   |

Explanation:

So, there are two tables. Now I want to find the following:

Given a USERID, Get sum of HIGH_COUNT & MEDIUM_COUNT. While counting the SUM it should be taken care that: If in USAGE, same device is used more than once, then the record which has the latest info (based on APPS.USAGE_START_TIME), should be considered while calculating the sum.

For ex:

For above schema, result should be (for userid=12) :

   | HIGH_COUNT (bigint)  | MEDIUM_COUNT (Bigint) |
    -----------------------------------------------
   |                356   |                   470 |

SQL Fiddle: http://sqlfiddle.com/#!2/74ae0f

Answer 1

If a user uses multiple APPS on one device, this query will use the APPS row with the highest usage_start_time :

select  a.userid
,       sum(u.high_count)
,       sum(u.medium_count)
from    apps a
join    `usage` u
on      u.appid = a.id
join    (
        select  u.device_id
        ,       a.userid
        ,       max(a.usage_start_time) as max_start_time
        from    apps a
        join    `usage` u
        on      u.appid = a.id
        group by
                u.device_id
        ,       a.userid
        ) filter
on      filter.device_id = u.device_id
        and filter.userid = a.userid
        and filter.max_start_time = a.usage_start_time
group by
        a.userid

In your dataset, it will select usage rows 5, 3, 4 for user 12 .

See it working at SQL Fiddle.

Answer 2

I can't quite get your numbers, but something like this should work...

SELECT a.userid
     , SUM(u.high_count)
     , SUM(u.medium_count)
  FROM apps a
  JOIN `usage` u
    ON u.appid = a.id
  JOIN 
     ( SELECT userid
            , deviceid
            , MAX(usage_start_time) max_usage_start_time
         FROM apps a
         JOIN `usage` u
           ON u.appid = a.id
        GROUP
           BY userid
            , deviceid
     ) x
    ON x.userid = a.userid
   AND x.deviceid = u.deviceid
   AND x.max_usage_start_time = a.usage_start_time
 GROUP
    BY userid;

Note that usage is a reserved word. Therefore, this is a bad name for a column (or a table). Also, note inconsistencies between your question and your fiddle.

Answer 3

I think not had chance to test it but

SELECT SUM(HIGH_COUNT), SUM(MEDIUM_COUNT) FROM `USAGE` INNER JOIN `APPS` ON USAGE.APPID=APPS.ID WHERE APPS.USERID=$input_user_id_to_lookup

will give you your counts.

For yoru other question (homework?) you didn't give us the full schema so we can't guess what you need doing.

Also whoever designed that db should be shot its horrible