SQL query over period of time

Question

I have a table in MySQL with some data of a temperature sensor. I have 1 row per minute. I would like to perform a query to know whether there was any span of 1 hour where the average temperature was greater than 4 degrees. I know how to perform a query where a single row was above 4:

SELECT * FROM sensor WHERE t>4

But in my case, I want to know whether there was a 1 hour period where the average temperature was over 4, not only a single row.

Not sure how to write such query...

Answer 1

One method is:

select s.*
from sensor s
where t > 4 and
      not exists (select 1
                  from sensor s2
                  where s2.datetime >= s.datetime and
                        s2.datetime < s.datetime + interval 1 hour and
                        s2.t <= 4
                 );

EDIT:

Arggh. The question asks about the average temperature, not any temperature (the question is quite clear, I just misread it).

Here is a variation to handle that:

select s.*,
       (select avg(s2.t)
        from sensor s2
        where s2.datetime >= s.datetime and
              s2.datetime < s.datetime + interval 1 hour 
       ) as avg_t
from t
having avg_t > 4;

This uses an extension to MySQL where the having clause can make use of a column alias for filtering.

Answer 2

Another method is:

SELECT *, 
    AVG(`t`) AS `avg_t`, 
    DATE_FORMAT(`date`, '%Y-%m-%d %H') AS `date_and_hour`
FROM `sensor`
GROUP BY DATE_FORMAT(`date`, '%Y-%m-%d %H')
HAVING `avg_t` > 4;

Answer 3

SELECT DATE_FORMAT(DATE_TIME_FIELD, '%Y-%m-%d %H') AS DATE_TIME_HOUR
FROM sensor
GROUP BY DATE_FORMAT(DATE_TIME_FIELD, '%Y-%m-%d %H')
HAVING AVG(T)>4