如何使用MySQL从max（datetime）获取对应的值？

Question

MySQL software table:

.------------------------------------------------------------.
| id | hostname   | software | version | install_date        |
|------------------------------------------------------------|
| 1  | computer-a | vim      | 1.00    | 2012-01-04 00:00:00 | 
|------------------------------------------------------------|
| 1  | computer-a | vim      | 2.00    | 2012-01-05 00:00:00 | 
|------------------------------------------------------------|
| 1  | computer-b | emacs    | 2.00    | 2012-01-04 00:00:00 | 
|------------------------------------------------------------|
| 1  | computer-b | emacs    | 1.00    | 2012-01-05 00:00:00 | 
|------------------------------------------------------------|
| 1  | computer-c | emacs    | 2.00    | 2012-01-05 00:00:00 | 
|------------------------------------------------------------|
| 1  | computer-c | vim      | 3.00    | 2012-01-05 00:00:00 | 
'------------------------------------------------------------'

MySQL query:

SELECT 
  hostname,
  MAX(IF(software = 'vim', version, NULL)) AS VIM,
  MAX(IF(software = 'emacs', version, NULL)) AS EMACS
FROM
  software
GROUP BY
  hostname;

Result:

+-------------+---------+----------+
| hostname    | VIM     | EMACS    |
+-------------+---------+----------+
| computer-a  | 2.0     | NULL     |
+-------------+---------+----------+
| computer-b  | NULL    | 2.0      |
+-------------+---------+----------+
| computer-c  | 3.0     | 2.0      |
+-------------+---------+----------+

http://www.sqlfiddle.com/#!8/6247d/3

The problem is that I want to show the versions with the most recent install_date for each software . Computer-b should be showing 1.0 for EMACS. How can the query be changed to accomplish this?

I can change the query to:

SELECT 
  hostname,
  MAX(IF(software = 'vim', install_date, NULL)) AS VIM,
  MAX(IF(software = 'emacs', install_date, NULL)) AS EMACS
FROM
  software
GROUP BY
  hostname;

This pulls the right install_date but how can I show the corresponding version ?

http://www.sqlfiddle.com/#!8/6247d/4

Answer 1

You can use a subquery to get the max(install_date) for each hostname and then join that back to your table:

select s1.hostname,
  MAX(IF(software = 'vim', version, NULL)) AS VIM,
  MAX(IF(software = 'emacs', version, NULL)) AS EMACS
from software s1
inner join
(
  select max(install_date) i_date,
    hostname
  from software
  group by hostname
) s2
  on s1.install_date = s2.i_date
  and s1.hostname = s2.hostname
group by s1.hostname

See SQL Fiddle with Demo . The result is:

|   HOSTNAME |    VIM |  EMACS |
--------------------------------
| computer-a |   2.00 | (null) |
| computer-b | (null) |   1.00 |
| computer-c |   3.00 |   2.00 |

You might need to group by both the hostname and software in the subquery if the software could have different install_date :

select s1.hostname,
  MAX(IF(s1.software = 'vim', version, NULL)) AS VIM,
  MAX(IF(s1.software = 'emacs', version, NULL)) AS EMACS
from software s1
inner join
(
  select max(install_date) i_date,
    hostname, software
  from software
  group by hostname, software
) s2
  on s1.install_date = s2.i_date
  and s1.hostname = s2.hostname
group by s1.hostname

See SQL Fiddle with Demo