Question

我有一个select语句，在每个“unit”只需要一个结果的情况下返回数千个结果

表

LID     unitnumb    lat         long           time          unitid  shift         unit  timeon               timeoff           crew1       crew2   care  onduty  randid
226733  cgrausam    36.7929227  -76.1340058 2014-11-28 21:41:51 388 pathfinderKZ,CH 22  2014-11-18 09:38:54 2014-11-18 09:38:54 kzavocki    cgrausam    NULL    1   IdXENPgZ
226730  kzavocki    40.6324391  -74.4208074 2014-11-28 21:41:43 301 pathfinderKAT   22  2014-10-27 14:28:02 2014-10-27 14:28:02 kzavocki    admin   NULL    1   gIahPbib
226732  kzavocki    40.6324391  -74.4208074 2014-11-28 21:41:43 409 pathfinderKAT   22  2014-11-21 10:33:32 2014-11-21 10:33:32 kzavocki    admin   NULL    1   JFASXDXC
226735  kzavocki    40.6324391  -74.4208074 2014-11-28 21:41:43 214 Rover-Kathy     26  2014-10-07 08:47:19 2014-10-07 08:47:19 kzavocki    admin   NULL    1   CLJSZXeW

我有一个select语句返回这些结果：

SELECT * 
FROM gpstracking g 
LEFT JOIN units u 
ON g.unitnumb = u.crew1 OR g.unitnumb = u.crew2 
WHERE u.onduty = 1 and time > DATE_SUB(NOW(), INTERVAL 2 HOUR) 
ORDER BY time DESC

目前这个select语句完全符合我的要求除了我只需要每个“单位”1个结果，我希望结果是最近的“时间” 所以理想的结果将是

226733  cgrausam    36.7929227  -76.1340058 2014-11-28 21:41:51 388 pathfinderKZ,CH 22  2014-11-18 09:38:54 2014-11-18 09:38:54 kzavocki    cgrausam    NULL    1   IdXENPgZ
226735  kzavocki    40.6324391  -74.4208074 2014-11-28 21:41:43 214 Rover-Kathy     26  2014-10-07 08:47:19 2014-10-07 08:47:19 kzavocki    admin   NULL    1   CLJSZXeW

从上面开始 - 如何修改select语句只显示每个“unit”的单个结果

Answer 1

一种方法是使用变量：

select *
from (select *,
             (@rn := if(@u = u.unit, @rn + 1,
                        if(@u := u.unit, 1, 1)
                       )
             ) as seqnum
      from gpstracking g LEFT JOIN
           units u
           on g.unitnumb = u.crew1 or g.unitnumb = u.crew2 CROSS JOIN
           (select @u := '', @rn := 0) vars
      where u.onduty = 1 and time > DATE_SUB(NOW(), INTERVAL 2 HOUR)
      order by u.unit, time desc
     ) ut
where seqnum = 1
order by time desc;

根据列内容选择单行结果

1 个答案: