difference in time is giving incorrect results - sql

Question

I have a table as test
shiftend | out |
---------------------------------------------
15:00:00.0000000 | 2016-07-22 14:42:00 |
16:00:00.0000000 | 2016-07-22 16:06:00 |
Shiftend is having a datatype as time
out is having a datatype as smalldatetime
I am expecting the output as
shiftend | out | Output
-----------------------------------------------------------------------------
15:00:00.0000000 | 2016-07-22 14:42:00 | -00:18:00
16:00:00.0000000 | 2016-07-22 16:06:00 | 00:06:00
I am trying this query:
select shiftend,out,CAST((out-Shiftend) as time(0)) as Output from test
where
CAST(CONVERT(NVARCHAR(10), out, 101) AS SMALLDATETIME) = CAST(CONVERT(NVARCHAR(10),'2016-07-22', 101) AS SMALLDATETIME)
But i am getting the output as
shiftend | out | Output
-----------------------------------------------------------------------------
15:00:00.0000000 | 2016-07-22 14:42:00 | 23:42:00
16:00:00.0000000 | 2016-07-22 16:06:00 | 00:06:00
23:42:00 is incorrect. How to calcualte the time.

Answer 1

Try the following query:
select
case when (cast(out as time) < shiftend) then '-' else '' end +
convert(varchar(8),
dateadd(minute,
abs(
DATEDIFF(minute,
cast(out as time)
, shiftend)
)
,0)
,108) as Output
Explanation:
You're getting the difference between the two dates with DATEDIFF(minute, cast(out as time), shiftend).
You need just the time component to avoid going to the previous day, so you use cast(out as time). shiftend as you mentioned is already of datatype time
abs returns the absolute value, so -18 becomes 18.
Then generate a date by adding the above value as minutes to 00:00:00 using dateadd(minute, [above value], 0)
The final convert(varchar(8),____,108) is since you required the output as a time.
iif(cast(out as time) < shiftend,'-','') adds negative sign or not to the beginning of the word.
Unfortunately, you can't have negative values in the time datatype. It's turning -18 into 00:00 - 18 which is 23:42. You could:
Use datediff and save the difference as the number of seconds for example.
Save only the modular difference and have a separate column for deciding whether it's a positive or negative difference.
Write your own SQL function for computing this as a varchar as above