如何使用 MySQL 从 max(datetime) 中获取相应的值？

Question

MySQL 软件表：

.------------------------------------------------------------.
| 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 查询：

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

结果：

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

问题是我想为每个software 显示具有最新install_date 的版本。 Computer-b 应该为 EMACS 显示 1.0。如何更改查询以完成此操作？

我可以将查询更改为：

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;

这会拉出正确的 install_date，但我怎样才能显示相应的 version？

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

Answer 1

您可以使用子查询为每个 hostname 获取 max(install_date)，然后将其连接回您的表：

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

见SQL Fiddle with Demo。结果是：

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

如果software 可能有不同的install_date，您可能需要group by 子查询中的hostname 和software：

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

见SQL Fiddle with Demo