SQL 查询根据几个条件查找持续时间

Question

I have a table named status with following data in it:

time       var  value
12:00:01    a   10
12:00:01    b   10
12:00:01    c   12
12:00:05    a   9
12:00:05    b   10
12:00:05    c   10
12:00:10    a   13
12:00:10    b   1
12:00:10    c   4
14:00:01    a   12
14:00:01    b   11
14:00:01    c   9
14:00:41    a   9
14:00:41    b   9
14:00:41    c   3

Now I want to find the duration for which A > 10 and B > 10 I want the answer as: 44 sec (duration from 12:00:01 to 12:00:05 = 4 sec, duration from 14:00:01 to 14:00:41 = 40 sec, total 44 sec) I tried:

SELECT sum(time) as duration FROM status WHERE A>10 AND B>10;

Answer 1

This is a gaps and islands problem, which can be solved using a cumulative sum. The lag() is used to determine where islands start. The cumulative sum defines groups.

SQL date/time functions are notoriously database-dependent. So the following gives the right idea, but it may not work directly in your database. So, the times when the condition occurs is returned by the following query:

select min(time), max(time)
from (select t.*,
             sum(case when a < 10 or b < 10 then 1 else 0 end) over (order by time) as grp
      from status t
     ) t
where a > 10 and b > 10;

The sum() is then an aggregation on this. For instance:

select sum(maxtime - mintime)
from (select max(time) as maxtime, min(time) as mintime
      from (select t.*,
                   sum(case when a < 10 or b < 10 then 1 else 0 end) over (order by time) as grp
            from status t
           ) t
      where a > 10 and b > 10
     ) t