如何使用MySQL从max(datetime)中获取相应的值?
MySQL软件表:如何使用MySQL从max(datetime)中获取相应的值?,mysql,database,pivot-table,Mysql,Database,Pivot Table,MySQL软件表: .------------------------------------------------------------. | id | hostname | software | version | install_date | |------------------------------------------------------------| | 1 | computer-a | vim | 1.00 | 2012-01-04
.------------------------------------------------------------.
| 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 |
+-------------+---------+----------+
问题是我想为每个软件
显示最新的安装日期
版本。对于EMACS,Computer-b应显示1.0。如何更改查询以实现这一点
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;
这将获取正确的安装日期,但如何显示相应的版本
您可以使用子查询获取每个
主机名的最大值(安装日期)
,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
看。结果是:
| HOSTNAME | VIM | EMACS |
--------------------------------
| computer-a | 2.00 | (null) |
| computer-b | (null) | 1.00 |
| computer-c | 3.00 | 2.00 |
如果软件
可能具有不同的安装日期
,则可能需要在子查询中对主机名
和软件
进行分组:
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
请参见一个问题,如果更改了其中一个computer-c安装的日期,则只能看到最新安装的版本@codybuell然后您将通过主机名和软件以及我的编辑将子查询更改为group