Question

我有一个MySQL数据库，结构如下......

网站有很多传感器传感器有许多SensorReadings

我想获取一个站点的所有传感器以及所有这些传感器的最后5个SensorReadings。我怀疑我将不得不对存储过程和临时表做一些事情（如果它们甚至存在于MySQL中。

Answer 1

SELECT reading,
       date
FROM   (select sensor_id,
               reading,
               date,
               @num := if(@sensor_id = sensor_id, @num + 1, 1) as row_number,
               @sensor_id := sensor_id                         as dummy
        from   sensor_readings
        order  by sensor_id,
                  date desc) T
WHERE  row_number<=5

请在您的问题中提供您的实际表格结构。

Answer 2

使用MySQL变量的完整示例。为简洁起见，这会显示每个传感器的前2个读数。

drop table if exists Sensors;
create table Sensors (Id int);
insert Sensors (id) values (1), (2), (3);

drop table if exists SensorReadings;
create table SensorReadings (SensorId int, RecordDate date);
insert SensorReadings (SensorId, RecordDate) values 
    (1, '2011-01-01'),
    (1, '2011-01-02'),
    (1, '2011-01-03'),
    (2, '2011-01-01'),
    (2, '2011-01-02'),
    (2, '2011-01-03');


set @num = -1;
set @SensorId = -1;

select  *
from    Sensors s
join    (
        select  *
        ,       @num := if(@SensorId = SensorId, @num + 1, 1) as rn
        ,       @SensorId := SensorId
        from    SensorReadings sr
        order by
                SensorId
        ,       RecordDate desc
        ) as numbered
on      numbered.SensorId = s.Id
where   numbered.rn < 3;

Answer 3

基于Andomar转储

select * from SensorReadings as t1
where (select count(*) from SensorReadings as t2
       where t1.sensorid = t2.sensorid and t2.recordDate > t1.recordDate) <2

最近5条多条记录的相关记录

3 个答案: