SQL Server grouping a timestamp by hour but keep as date format (don't want to pull hour out) - sql

Question

When I want to group a bunch of time stamps by day, by
CONVERT (datetime, CONVERT (varchar, dbo.MEASUREMENT_Battery.STAMP, 101))
it produces for me a "day" stamp that SQL Server still views as a date and can be sorted and used as such.
What I'm trying to figure out is if it's possible to do the same thing by hour. I tried
CAST(DATEPART(Month, STAMP) AS varchar) + '/' + CAST(DATEPART(Day, STAMP) AS varchar) + '/' + CAST(DATEPART(Year, STAMP) AS varchar) + ' ' + CAST(DATEPART(Hour, STAMP) AS varchar) + ':00:00.000'
and this "works" but SQL Server doesn't view this as a date anymore so I can't sort properly.
The end result I want is right though: ex: 9/9/2015 9:00:00.000

Answer 1

Do NOT convert into a string, until you absolutely have to "present" the result.
CONVERT() or FORMAT() return string representations of temporal information
The following method returns a datetime value truncated to the hour without resorting to string manipulation (and hence fast).
select
dateadd(hour, datediff(hour,0, dbo.MEASUREMENT_Battery.STAMP ), 0)
, count(*)
from dbo.MEASUREMENT_Battery
group by
dateadd(hour, datediff(hour,0, dbo.MEASUREMENT_Battery.STAMP ), 0)
SQL Fiddle
MS SQL Server 2014 Schema Setup:
CREATE TABLE MEASUREMENT_Battery
([STAMP] datetime)
;
INSERT INTO MEASUREMENT_Battery
([STAMP])
VALUES
('2015-11-12 07:40:15'),
('2015-11-12 08:40:15'),
('2015-11-12 09:40:15'),
('2015-11-12 10:40:15'),
('2015-11-12 11:40:15'),
('2015-11-12 12:40:15'),
('2015-11-12 13:40:15'),
('2015-11-12 14:40:15')
;
NOTE: the output below for column [Stamp] is the default display
Results:
| | |
|----------------------------|---|
| November, 12 2015 07:00:00 | 1 |
| November, 12 2015 08:00:00 | 1 |
| November, 12 2015 09:00:00 | 1 |
| November, 12 2015 10:00:00 | 1 |
| November, 12 2015 11:00:00 | 1 |
| November, 12 2015 12:00:00 | 1 |
| November, 12 2015 13:00:00 | 1 |
| November, 12 2015 14:00:00 | 1 |
If you absolutely insist on dipay of a date/time value a paricular way, then you may add the display format in the select clause (but not needed in the group by clause!)
select
FORMAT(dateadd(hour, datediff(hour,0, dbo.MEASUREMENT_Battery.STAMP ), 0) , 'MM/dd/yyyy HH')
, count(*)
from dbo.MEASUREMENT_Battery
group by
dateadd(hour, datediff(hour,0, dbo.MEASUREMENT_Battery.STAMP ), 0)

Answer 2

What happens is that when you use the DateTime Style 101 (at the end of the second CONVERT) the Date will be converted to mm/dd/yyyy and the time to 00:00:00.000 always as stated here an:
https://msdn.microsoft.com/en-us/library/ms187928.aspx
Now, from what I understand from your question is that you would like to include the hour as well and this can be done like this:
SELECT FORMAT(STAMP , 'MM/dd/yyyy HH') + ':00:00.000'
Note:
':00:00.000' is optional and is just for a nicer output.
This only works in SQL Server 2012 and later version.
Testing with some test date we will see that we get the expected result:
-- Drop temp table if it exists
IF OBJECT_ID('tempdb..#T') IS NOT NULL DROP TABLE #T
-- Create temp table
CREATE TABLE #T ( myDate DATETIME )
-- Insert dummy values
INSERT INTO #T VALUES ( '2015-12-25 14:00:00.000' ) -- 14
INSERT INTO #T VALUES ( '2015-12-25 14:00:00.000' ) -- 14
INSERT INTO #T VALUES ( '2015-12-25 15:00:00.000' )
INSERT INTO #T VALUES ( '2015-12-25 16:00:00.000' )
INSERT INTO #T VALUES ( '2015-12-25 17:00:00.000' ) -- 17
INSERT INTO #T VALUES ( '2015-12-25 17:00:00.000' ) -- 17
-- Select query
SELECT COUNT( myDate ), MAX( FORMAT( myDate , 'MM/dd/yyyy HH') + ':00:00.000' ) FROM #T
GROUP BY DATEPART( hour, myDate )
Output:
2 12/25/2015 14:00:00.000
1 12/25/2015 15:00:00.000
1 12/25/2015 16:00:00.000
2 12/25/2015 17:00:00.000