Question

users:

user_id    user_name     
---------------------

 1          User A
 2          User B

tracking:

user_id     track         
---------------------

 1           no
 2           no

applications:

user_id    date_of_application   date_ended    grade    status    
---------------------------------------------------------------

 1            2011-01-01         2011-02-28     1.0     Ended
 1            2011-02-02         2011-03-28     1.0     Ended
 1            2011-03-03         2011-04-28     (1.5)   Ended

 2            2011-01-01         2011-02-20     2.0     Ended
 2            2011-02-02         2011-03-11     2.5     Ended
 2            2011-03-03         2011-04-28     (1.0)   Ended

 1            2011-05-10              -          -      Pending
 2            2011-05-15              -          -      Pending

• note that the table can contain multiple records of the same user as long as all its previous applications have ended (status = ended)
• user_id is not unique (applies to the applications table only)
• date is in yy-mm-dd format
• date_ended and grade are only updated the instant the application has ended
• also, I understand that it probably is recommended for ‘status’ to have its own table, however I would prefer that the above tables are taken as is (minus the typos and significant errors of course)

What I want to accomplish here is to retrieve all rows WHERE status is ‘Pending’ and such that the value for the grade column for each of these retrieved rows is the value of the latest grade (in other words the row with the latest date_ended), (in parenthesis above) where status is ‘Ended’ for this particular user (or row).

Also, I would need to have the first 10 rows of the result to be ORDERed BY grade ASC. And have the succeeding rows after that (11th row up to the final row) to be ORDERed BY date_of_application ASC.

Clearly SQL queries isn’t my strongest area so I’m not sure if it’s better (or is only possible) to perform those ORDER BY(s) using 2 or more queries. I however prefer this to be done using a single query only.

The desired result:

user_id   user_name   date_of_application   grade   status   track 
--------------------------------------------------------------------

 1         User A         2011-05-10        (1.5)    Pending    no
 2         User B         2011-05-15        (1.0)    Pending    no

Working code I have so far on my end [minus the possible typos], (and listed are additions to be applied):

• latest grade
• ORDER BY grade (first 10), ORDER BY date_of_application (11th up to last row)

Query:

SELECT users.user_name,
       t.track,
       a.user_id,
       a.date_of_application,
       a.status,
       (SELECT ae.grade
          FROM applications AS ae
          WHERE ae.status = 'Ended' 
            AND ae.user_id = a.user_id                                                            
         LIMIT 1) AS grade
  FROM users
  JOIN applications AS a ON users.user_id = a.user_id
  JOIN tracking AS t ON users.user_id = t.user_id
 WHERE a.status = 'Pending'
ORDER BY grade ASC

Answer 1

You probably trying to do too much in one query here.

Anyway, if you want something to hurt your eyes:

 select a.* from
 (
 SELECT u.user_name,
 a.user_id,
 a.date_of_application,
 td.grade,
 a.status,
 t.track
 FROM users u
 JOIN applications AS a ON u.user_id = a.user_id
 JOIN tracking AS t ON u.user_id = t.user_id
 LEFT OUTER JOIN
 (
 select ap.user_id,ap.grade 
 from applications ap
 inner join
 (select a.user_id,max(date_ended) as max_ended_date
 from applications a
 where a.status = 'Ended'
 group by a.user_id
 ) md on md.user_id = ap.user_id and ap.date_ended = md.max_ended_date
 ) as td on u.user_id = td.user_id
 WHERE a.status = 'Pending'
 ORDER BY cast(replace(replace(td.grade,'(',''),')','') as decimal(12,2)),u.user_id ASC
 LIMIT 10
 ) a
 WHERE grade is not null
 UNION ALL
 select b.* from
 (
 SELECT u.user_name,
   u.user_id,
   a2.date_of_application,
   td.grade,
   ifnull(a2.status,'No applications yet') as status,
   t2.track
 FROM users u
 LEFT OUTER JOIN (select user_id,date_of_application,status from applications where     status = 'Pending') AS a2 ON u.user_id = a2.user_id
 JOIN tracking AS t2 ON u.user_id = t2.user_id
 LEFT OUTER JOIN
 ( 
 select ap.user_id,ap.grade 
 from applications ap
 inner join
  (select a.user_id,max(date_ended) as max_ended_date
  from applications a
  where a.status = 'Ended'
  group by a.user_id
 ) md on md.user_id = ap.user_id and ap.date_ended = md.max_ended_date
 ) as td on u.user_id = td.user_id
 where u.user_id not in (
 select t1.user_id
 from (
 select ap1.user_id,ap1.grade 
  from applications ap1
  inner join
  (select a1.user_id,max(date_ended) as max_ended_date
   from applications a1
   where a1.status = 'Ended'
   group by a1.user_id
  ) md1 on md1.user_id = ap1.user_id and ap1.date_ended = md1.max_ended_date
  order by cast(replace(replace(ap1.grade,'(',''),')','') as decimal(12,2)),md1.user_id asc
  limit 10
  ) as t1
 )
 ORDER BY status desc,a2.date_of_application ASC
 ) b;

This does make the following assumptions:

1. There is always only one row for each user_id in the users and
   tracking table

EDIT

To explain this query a bit:

Inline view aliased a (aka ‘The Top Half’) brings back a list of the top 10 users according to their most recent ‘ended’ grade ascending. Note the following part of the query that strips any brackets from the grade, converts the resulting number to a decimal to 2 decimal places and orders them ascending by grade and then, in case of equal grade scores, by user_id:

ORDER BY cast(replace(replace(td.grade,'(',''),')','') as decimal(12,2)),u.user_id ASC

Inline view b is pretty much the same as inline view a except that excludes users that would appear in The Top Half and orders the results by status DESC (to move those users with no applications to the bottom of the list) and date of application ASC.