mysql - 如何从最近的记录中选择值？

Question

我有 table

create table events (
  sensor_id integer not null,
  event_type integer not null,
  value integer not null,
  time timestamp unique not null);

insert into events
values
(2,2,5,"2014-02-13 12:42:00"),
(2,4,-42,"2014-02-13 13:19:57"),
(2,2,2,"2014-02-13 14:48:30"),
(3,2,7,"2014-02-13 12:54:39"),
(2,3,54,"2014-02-13 13:32:36");

我只希望每个 sensor_id 和 event_type 的最新值在时间方面

预期结果

sensor_id, event_type   value
2          2            2
2          3            54
2          4            -42
3          2            7

我试过查询

SELECT sensor_id,event_type,value, time
from events
group by sensor_id,event_type
having time = max(time)

为什么它不成功(结果表不包含 sensor_id = 2 和 event_type = 2)

我如何同时使用 mysql 和 sqllite 来解决这个问题，这方面有区别吗？

Answer 1

使用聚合来获取最大时间，然后使用连接来获取与该时间匹配的其余记录:

SELECT e.sensor_id, e.event_type, e.value, e.time
from events e join
     (select sensor_id, event_type, max(time) as maxt
      from events e
      group by sensor_id, event_type
     ) ee
     on e.sensor_id = ee.sensor_id and e.event_type = ee.event_type;