Add to DATE hour - TSQL - date

Question

I have a column type time(7).
What i want is to add in time column to date.
i manage to get the only date using GETDATE() function but i fail to add the time part next to date.
Query:
SELECT [Compay]
,[Time]
,CAST(GETDATE() AS DATE) AS Today
,CAST(CAST(GETDATE() AS DATE) AS NVARCHAR) AS Today_AS_Nvarchar
,CAST([Time] AS NVARCHAR) AS Time_AS_Nvarchar
,CAST(CAST(GETDATE() AS DATE) AS NVARCHAR) + ' ' + CAST([Time] AS NVARCHAR) AS Today_Time_AS_Nvarchar
,CONVERT(datetime,CAST(CAST(GETDATE() AS DATE) AS NVARCHAR) + ' ' + CAST([Time] AS NVARCHAR),103)
FROM [Testing_Env].[dbo].[Com_CD_Test]
Error:
Conversion failed when converting date and/or time from character string.
The error arise on CONVERT(datetime,CAST(CAST(GETDATE() AS DATE) AS NVARCHAR) + ' ' + CAST([Time] AS NVARCHAR),103)
is there any easier/orthodox way to achieve it?

Answer 1

You can't add the new date and time data types together like you can the old data types; personally I think this is also better as it stop people treating dates and times like a numerical value.
Assuming you have a date column and a time column you have a few of options. The first is to CAST/CONVERT both to a datetime and then "add" them together. Because the "old" data types work more like numerical values this works, however, you will lose accuracy of your time value if it has a precision of 3 or higher (as datetime is only accurate to 1/300 seconds):
DECLARE #TimeValue time(7) = '17:52:12.1234567',
#DateValue date = '20211016';
SELECT CONVERT(datetime, #DateValue) + CONVERT(datetime, #TimeValue);
If loosing accuracy isn't an option, then you could to use conversion on the date value and use DATEDIFF and DATEADD. For a time(7) you'll want to be using nanoseconds (as microseconds isn't accurate enough). Unfortunately this poses another problem; DATEADD can't handle bigint values (still) and there is no DATEADD_BIG (like there is DATEDIFF_BIG), so this becomes overly complex. You need to first get the difference in milliseconds, and then add the remainder in nanoseconds to still be accurate to 1/1000000 of a second:
DECLARE #TimeValue time(7) = '17:52:12.1234567',
#DateValue date = '20211016';
SELECT DATEADD(NANOSECOND,DATEDIFF_BIG(NANOSECOND,'00:00:00', #TimeValue) % 1000000,DATEADD(MILLISECOND, DATEDIFF_BIG(MILLISECOND,'00:00:00', #TimeValue), CONVERT(datetime2(7),#DateValue)));
Finally, yes, you can convert to values to strings, and then to a datetime2 value; this is probably the easiest methiod. You just need to ensure you use style codes:
DECLARE #TimeValue time(7) = '17:52:12.1234567',
#DateValue date = '20211016';
SELECT CONVERT(datetime2(7),CONVERT(varchar(10), #DateValue, 23) + 'T' + CONVERT(varchar(17), #TimeValue, 114),126);