Question

考虑下表：

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

我想在SQL中创建一个PostgreSQL - 查询，该查询提取温度高于50°C的行至少30分钟，理想情况下从何时开始知道温度是多少实际上高于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所需信息：

class Post(ndb.Model):
  name = ndb.StringProperty()
  votes = ndb.IntegerPropery()

Answer 2

Gordon的解决方案可以简化为单个OLAP功能：

select min(datetime), max(datetime), count(*) as numrecs, avg(temperature)
from
 (
   select datetime, temperature, 
      -- previous time when temperature was < 50
      -- same time for all rows with a temp >= 50
      max(case when temperature < 50 then datetime end)
      over (order by datetime
            rows unbounded preceding) as prevlow
   from measurement
 ) as dt
where temperature >= 50
group by prevlow
having max(datetime) >= min(datetime) + interval '30' minute;

SQL在特定时间内查找高于阈值的值

2 个答案: