SQL在特定时间内查找高于阈值的值

Question

请考虑下表:

create table measurement (
    datetime     timestamp,
    temperature  numeric(5,2)
);

我想创建一个SQL-query,PostgreSQL其中提取温度高于50°C的行至少30分钟,理想情况下从何时知道温度实际高于50°C.示例数据如下:

datetime               temperature
-------------------    -----------
2017-03-15 19:00:10    49.56
2017-03-15 19:15:10    52.81
2017-03-15 19:30:10    49.00
2017-03-15 19:45:10    52.88
2017-03-15 20:00:10    49.56
2017-03-15 20:15:10    49.13
2017-03-15 20:30:10    51.31   <--
2017-03-15 20:45:10    52.06   <--
2017-03-15 21:00:10    50.50   <--
2017-03-15 21:15:10    50.50   <--
2017-03-15 21:30:10    49.38
2017-03-15 21:45:10    47.44
2017-03-15 22:00:10    46.19
2017-03-15 22:15:10    45.44
2017-03-15 22:30:10    50.25
2017-03-15 22:45:10    48.56
2017-03-15 23:00:10    51.25   <--
2017-03-15 23:15:10    50.44   <--
2017-03-15 23:30:10    50.63   <--
2017-03-15 23:45:10    46.75

Answer 1

因此,温度高于50的第一个身份组.这是一个缺口和岛屿问题.然后,您可以汇总岛屿以获取您想要的信息:

select min(datetime), max(datetime), count(*) as numrecs, avg(temperature)
from (select t.*,
             row_number() over (order by datetime) as seqnum,
             row_number() over (partition by (temperature >= 50)::int 
                                order by datetime) as seqnum_t
      from t
     ) t
where temperature >= 50
group by (seqnum - seqnum_t)
having max(datetime) >= min(datetime) + interval '30' minute;