在一个表中选择行，在其他相关表中添加行的MAX（Date）行

Question

I have a table containing a set of tasks to perform:

Task

    ID  Name

    1   Washing Up
    2   Hoovering
    3   Dusting

The user can add one or more Notes to a Note table. Each note is associated with a task:

Note

    ID  ID_Task     Completed(%)    Date

    11  1       25      05/07/2013 14:00
    12  1       50      05/07/2013 14:30
    13  1       75      05/07/2013 15:00
    14  3       20      05/07/2013 16:00
    15  3       60      05/07/2013 17:30

I want a query that will select the Task ID, Name and it's % complete, which should be zero if there aren't any notes for it. The query should return:

    ID  Name        Completed (%)

    1   Washing Up  75
    2   Hoovering    0
    3   Dusting     60

I've really been struggling with the query for this, which I've read is a "greatest n per group" type problem, of which there are many examples on SO, none of which I can apply to my case (or at least fully understand). My intuition was to start by finding the MAX(Date) for each task in the note table:

SELECT  ID_Task,
            MAX(Date) AS Date
            FROM
            Note
            GROUP BY
            ID_Task

Annoyingly, I can't just add "Complete %" to the above query unless it's contained in a GROUP clause. Argh! I'm not sure how to jump through this hoop in order to somehow get the task table rows with the column appended to it. Here is my pathetic attempt, which fails as it only returns tasks with notes and then duplicates task records at that (one for each note, so it's a complete fail).

SELECT  Task.ID,
    Task.Name,
    Note.Complete
    FROM
    Task        
    JOIN
    (SELECT ID_Task,
        MAX(Date) AS Date
        FROM
        Note
        GROUP BY
        ID_Task) AS InnerNote
    ON
    Task.ID = InnerNote.ID_Task
    JOIN
    Note
    ON
    Task.ID = Note.ID_Task

Can anyone help me please?

Answer 1

If we assume that tasks only become more complete, you can do this with a left outer join and aggregation:

select t.ID, t.Name, coalesce(max(n.complete), 0)
from tasks t left outer join
     notes n
     on t.id = n.id_task
group by t.id, t.name

If tasks can become "less complete" then you want the one with the last date. For this, you can use row_number() :

select t.ID, t.Name, coalesce(n.complete, 0)
from tasks t left outer join
     (select n.*, row_number() over (partition by id_task order by date desc) as seqnum
      from notes n
     ) n
     on t.id = n.id_task and n.seqnum = 1;

In this case, you don't need a group by , because the seqnum = 1 performs the same role.

Answer 2

How about this just get the max of completed and group by taskid

SELECT  t.ID_Task as ID,n.`name`,MAX(t.completed) AS completed
FROM `task` t RIGHT JOIN `note` n on (  t.ID_Task=n.ID )
 GROUP BY t. ID_Task

OR

SELECT  t.ID_Task as ID,n.`name`,
(CASE when MAX(t.completed) IS NULL THEN '0' ELSE  MAX(t.completed))AS completed
FROM `task` t RIGHT JOIN `note` n on (  t.ID_Task=n.ID )
 GROUP BY t. ID_Task

Answer 3

select a.ID, 
       a.Name, 
       isnull((select completed 
                 from Note 
                where ID_Task = b.ID_Task 
                  and Date = b.date),0)
from Task a 
LEFT OUTER JOIN (select ID_Task, 
                        max(date) date 
                   from Note 
               group by ID_Task) b
ON a.ID = b.ID_Task;

See DEMO here