MySQL select MAX(datetime) not returning max value

问题:

Example table:

id   computer  app      version     build    date
---|---------|------|------------|-------|---------
1  |  aaaa1  | app1 |   1.0.0    |   1   | 2013-11-11 09:51:07
2  |  aaaa1  | app2 |   2.0.0    |   2   | 2013-11-12 09:51:07
5  |  xxxx2  | app1 |   1.0.0    |   1   | 2013-11-13 09:51:07
3  |  cccc3  | app2 |   3.1.0    |   1   | 2013-11-14 09:51:07
4  |  xxxx2  | app1 |   1.0.0    |   2   | 2013-11-15 09:51:07
5  |  cccc3  | app2 |   3.1.1    |   3   | 2013-11-16 09:51:07
6  |  xxxx2  | app1 |   1.0.2    |   1   | 2013-11-17 09:51:07
7  |  aaaa1  | app1 |   1.0.2    |   3   | 2013-11-18 09:51:07

Desired output (not exact format or listing order), getting latest install for each app on each computer:

7. aaaa1 - app1 - 1.0.2 - 3 - 2013-11-18 09:51:07
2. aaaa1 - app2 - 2.0.0 - 2 - 2013-11-12 09:51:07
6. xxxx2 - app1 - 1.0.2 - 1 - 2013-11-17 09:51:07
5. cccc3 - app2 - 3.1.1 - 3 - 2013-11-16 09:51:07

My SQL statement:

SELECT 
        id,
        computer, 
        app, 
        version, 
        build, 
        MAX(date) AS installed
    FROM 
        data 
    WHERE 
        placement = 'xxx'
    GROUP BY 
        app, computer
    ;

This gives me:

1. aaaa1 - app1 - 1.0.0 - 1 - 2013-11-11 09:51:07

and not

7. aaaa1 - app1 - 1.0.2 - 3 - 2013-11-18 09:51:07

as I expected.

MAX(date) works if I ONLY select MAX(date) and nothing else. But then I don't get any data to work with (just latest date).

SELECT 
        MAX(date) AS installed

I'm not an SQL ninja so I will soon go bald by scratching my head because of this.

回答1:

Try like this:

SELECT d.id, d.computer, d.app, d.version, d.build, a.installed
FROM data d
INNER JOIN (
  SELECT computer, app, max(DATE) AS installed
  FROM data
  GROUP BY computer, app
  ) a ON a.computer = d.computer AND a.app = d.app
WHERE placement = 'xxx'

The inner query is getting you the max(date) for each pair of computer and app, then you just join with that to get the rest of the information.

回答2:

Try by casting the Datetime field

 SELECT 
            id,
            computer, 
            app, 
            version, 
            build, 
            MAX(cast(date as Datetime)) AS installed
        FROM 
            data 
        WHERE 
            placement = 'xxx'
        GROUP BY 
           app, computer, id, version, build
        ;

回答3:

max - is an aggregate function try to add all columns from select statement to GROUP BY:

GROUP BY 
    app, computer, id, version, build.

回答4:

It might be because you store your date as String, and comparing string act differently than comparing integer. You should store your date in unix Timestamp format, and they will be much easier to compare. But will need an extra effort to be displayed as normal English date.

回答5:

Whats wrong with:

SELECT 
    id,
    computer, 
    app, 
    version, 
    build, 
    `date` AS installed
FROM 
    data 
WHERE 
    placement = 'xxx'
ORDER BY installed DESC
GROUP BY app;

回答6:

MAX did not work for me, what worked was additional subquery where I preordered table by date:

SELECT d.id, d.computer, d.app, d.version, d.build, a.installed
FROM data d
INNER JOIN (
  SELECT computer, app, date as installed
  FROM (
    SELECT computer, app, date
    FROM data
    ORDER BY date desc
  ) as t
  GROUP BY computer, app
  ) a ON a.computer = d.computer AND a.app = d.app
WHERE placement = 'xxx'