如何使用MySQL從max（datetime）獲取對應的值？

Question

MySQL軟件表：

.------------------------------------------------------------.
| 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查詢：

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

結果：

+-------------+---------+----------+
| 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

問題是我想顯示每個software最新install_date版本。 Computer-b對於EMACS應該顯示1.0。 如何更改查詢以實現此目的？

我可以將查詢更改為：

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;

這可以拉正確的install_date，但是如何顯示相應的version ？

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

Answer 1

您可以使用子查詢來獲取每個hostname的max(install_date) ，然后將其重新加入表中：

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

請參閱帶有演示的SQL Fiddle 。 結果是：

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

如果software可以具有不同的install_date則可能需要在子查詢中對hostname和software進行group by ：

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

參見帶有演示的SQL Fiddle