SQL Query time spent between certain value - sql-server

Question

I have a database for all temperatures the last 10 years.
Now I want to find all periods where the temperature was above ex. 15 degree.
Simplified example:
...
2015-05-10 12
2015-05-11 15 |
2015-05-12 16 |
2015-05-13 17 |
2015-05-14 16 |
2015-05-15 15 |
2015-05-16 12
2015-05-17 11
2015-05-18 15 |
2015-05-19 12
2015-05-20 18 |
...
Så now I want get all time periods like this:
Min Max
2015-05-11 2015-05-15
2015-05-18 2015-05-18
2015-05-20 2015-05-20
Any suggestion of how this query will look like ?

Answer 1

You could use CTE
CREATE TABLE #Date (DateT datetime, Value int )
INSERT INTO #Date
VALUES ('2015-05-10',12),
('2015-05-11',15),
('2015-05-12',16),
('2015-05-13',17),
('2015-05-14',16),
('2015-05-15',15),
('2015-05-16',12),
('2015-05-17',11),
('2015-05-18',15),
('2015-05-19',12),
('2015-05-20',18)
WITH t AS (
SELECT DateT d,ROW_NUMBER() OVER(ORDER BY DateT) i
FROM #Date
WHERE Value >= 15
GROUP BY DateT
)
SELECT MIN(d) as DataStart,MAX(d) as DataFinal, ROW_NUMBER() OVER(ORDER BY DATEDIFF(day,i,d)) as RN
FROM t
GROUP BY DATEDIFF(day,i,d)
RN column is optional you could use
SELECT MIN(d) as DataStart,MAX(d) as DataFinal
FROM t
GROUP BY DATEDIFF(day,i,d)

Answer 2

Here is a solution using a gaps and islands algorithm. It looks kind of bulky but it runs fast and scales great. It is also modular if you want to add a gap-allowed parameter and you can rewrite it to partition by some other columns and it still performs nicely.
Inspired by Peter Larssons post here: http://www.sqltopia.com/?page_id=83
WITH [theSource](Col1,Col2)
AS
(
SELECT Col1,Col2 FROM (VALUES
('2015-05-10',12),
('2015-05-11',15),
('2015-05-12',16),
('2015-05-13',17),
('2015-05-14',16),
('2015-05-15',15),
('2015-05-16',12),
('2015-05-17',11),
('2015-05-18',15),
('2015-05-19',12),
('2015-05-20',18)
) as x(Col1,Col2)
)
,filteredSource([Value])
AS
(
SELECT Col1 as [Value]
FROM theSource WHERE Col2 >= 15
)
,cteSource(RangeStart, RangeEnd)
AS (
SELECT RangeStart,
CASE WHEN [RangeStart] = [RangeEnd] THEN [RangeEnd] ELSE LEAD([RangeEnd]) OVER (ORDER BY Value) END AS [RangeEnd]
FROM (
SELECT [Value],
CASE
WHEN DATEADD(DAY,1,LAG([Value]) OVER (ORDER BY [Value])) >= [Value] THEN NULL
ELSE [Value]
END AS RangeStart,
CASE
WHEN DATEADD(DAY,-1,LEAD([Value]) OVER (ORDER BY [Value])) <= [Value] THEN NULL
ELSE [Value]
END AS RangeEnd
FROM filteredSource
) AS d
WHERE RangeStart IS NOT NULL
OR RangeEnd IS NOT NULL
)
SELECT RangeStart AS [Min],
RangeEnd AS [Max]
FROM cteSource
WHERE RangeStart IS NOT NULL;