Getting minimum value at a given time in SQL - sql

Question

I have the following SQL table:
start_time end_time value
2016-01-01 00:00:00 2016-01-01 08:59:59 1
2016-01-01 06:00:00 2016-01-01 14:59:59 2
2016-01-01 12:00:00 2016-01-01 17:59:59 1.5
2016-01-01 03:00:00 2016-01-01 17:59:59 3
I want to convert it into:
start_time end_time min_value
2016-01-01 00:00:00 2016-01-01 08:59:59 1
2016-01-01 09:00:00 2016-01-01 11:59:59 2
2016-01-01 12:00:00 2016-01-01 17:59:59 1.5
where min_value is the minimum value at a given point in time. Is it possible to do this in SQL?

Answer 1

Try below. I think it does exactly what you asked
As you can see - I added one more entry in your example to make it a little spicier :o)
WITH YourTable AS (
SELECT TIMESTAMP '2016-01-01 00:00:00' AS start_time, TIMESTAMP '2016-01-01 08:59:59' AS end_time, 1 AS value UNION ALL
SELECT TIMESTAMP '2016-01-01 06:00:00' AS start_time, TIMESTAMP '2016-01-01 14:59:59' AS end_time, 2 AS value UNION ALL
SELECT TIMESTAMP '2016-01-01 12:00:00' AS start_time, TIMESTAMP '2016-01-01 17:59:59' AS end_time, 1.5 AS value UNION ALL
SELECT TIMESTAMP '2016-01-01 03:00:00' AS start_time, TIMESTAMP '2016-01-01 17:59:59' AS end_time, 3 AS value UNION ALL
SELECT TIMESTAMP '2016-01-01 12:30:00' AS start_time, TIMESTAMP '2016-01-01 12:40:59' AS end_time, 1 AS value
),
Intervals AS (
SELECT iStart AS start_time, LEAD(iStart) OVER(ORDER BY iStart) AS end_time
FROM (
SELECT DISTINCT iStart FROM (
SELECT start_time AS iStart FROM YourTable UNION ALL
SELECT end_time AS iStart FROM YourTable )
)
),
Intervals_Mins AS (
SELECT b.start_time, b.end_time, MIN(value) AS min_value
FROM YourTable AS a
JOIN Intervals AS b
ON b.start_time BETWEEN a.start_time AND a.end_time
AND b.end_time BETWEEN a.start_time AND a.end_time
GROUP BY b.start_time, b.end_time
),
Intervals_Group AS (
SELECT start_time, end_time, min_value, IFNULL(SUM(flag) OVER(PARTITION BY CAST(min_value AS STRING) ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING), 0) AS time_group
FROM (
SELECT start_time, end_time, min_value, IF(end_time = LEAD(start_time) OVER(PARTITION BY CAST(min_value AS STRING) ORDER BY start_time), 0, 1) AS flag
FROM Intervals_Mins
)
)
SELECT MIN(start_time) AS start_time, MAX(end_time) AS end_time, min_value
FROM Intervals_Group
GROUP BY min_value, time_group
-- ORDER BY start_time

Answer 2

Hmmm . . . This seems hard. I think the following strategy will work:
Break the data into two parts, for start times and end times.
For each start time calculate the minimum value in effect at that time.
For each end time, calculate the minimum value in effect starting at that time.
Recombine using a gaps-and-islands approach
I'm just not 100% sure you can do this in BQ, because it involves non-equijoins. But . . .
with starts as (
select start_time as time,
(select min(t2.value)
from t t2
where t.start_time between t2.start_time and t2.end_time
) as value
from t
),
ends as (
select end_time as time,
(select min(t2.value)
from t t2
where t2.end_time > t.end_time and
t2.start_time <= t.end_time
) as value
from t
)
select value, min(time), max(time)
from (select time,
row_number() over (order by time) as seqnum,
row_number() over (partition by value order by time) as seqnum_v
from ((select s.* from starts) union all
(select e.* from ends)
) t
) t
group by value, (seqnum - seqnum_v);

Answer 3

I'm not sure that I understand how the expected output relates to the input, but if you just want to associate the minimum value with distinct (start_time, end_time) pairs, you can do e.g.:
#standardSQL
WITH T AS (
SELECT TIMESTAMP '2016-01-01 00:00:00' AS start_time,
TIMESTAMP '2016-01-01 08:59:59' AS end_time, 1 AS value UNION ALL
SELECT TIMESTAMP '2016-01-01 06:00:00',
TIMESTAMP '2016-01-01 14:59:59', 2 UNION ALL
SELECT TIMESTAMP '2016-01-01 12:00:00',
TIMESTAMP '2016-01-01 17:59:59', 1.5 UNION ALL
SELECT TIMESTAMP '2016-01-01 3:00:00',
TIMESTAMP '2016-01-01 17:59:59', 3
)
SELECT
start_time,
end_time,
MIN(value) AS min_value
FROM T
GROUP BY start_time, end_time;