calculating duration (Date - Date) from a case Expression - sql

Question

I'm writing a complex query and I'm having some problems with calculating from case expression (thank you). Essentially, thanks to some great help on this site I managed to solve another issue by using an ordered table but now one of the select statements I was previously using no longer functions with the OT.
I get an error:
Conversion failed when converting date and/or time from character string
The core of the problem is that I want to calculate duration from subtracting a start date from an end date. The code I had previously used was
select
cast((CASE
WHEN ap.startLIKE '%[^0-9]%'
THEN CAST(ap.end as datetime)
ELSE NULL
END)
-
(CASE
WHEN ap.start LIKE '%[^0-9]%'
THEN CAST(ap.start as datetime)
ELSE NULL
END) as time) as Duration,
ap.start, ap.end, ap.description
from
Table as AP
group by
ap.start, ap.end, ap.description
but when I changed to draw this information from the ordered table as:
Select
ot.starttime as "Start Time",
ot.endtime as "End Time",
ot.description "Description",
ot.entrynumber as "ID Number",
cast((CASE
WHEN ot.endtime LIKE '%[^0-9]%'
THEN CAST(ot.endtime as datetime)
ELSE NULL
END)
-
(CASE
WHEN ot.starttime LIKE '%[^0-9]%'
THEN CAST(ot.starttime as datetime)
ELSE NULL
END) as time) as duration
From
OrderedTable as OT
group by
ot.starttime, ot.endtime, ot.description, ot.entrynumber
order by
ot.endtime
I get the error. Previously applying the group by clause had removed the error, but not from the ordered table.
Genuinely unsure what is causing the problem as I've been trying to eliminate the conversion to date issue constantly, I would appreciate any help.
The originating problem has been that the date fields on the server are in nvarchar format and not datetime, I've been trying to get around that.

Answer 1

If I understand your question correctly - you can use
TIMEDIFF(date1, date2)
or TIMESTAMPDIFF(SECOND, date2, date1);
where date2 < date1 to find the time between two dates. In the first case you will get the result in HH:mm:ss format and in the second case it will be in seconds - depends what you need. Since you didn't post an example how a date looks in your column I can't be sure, but I think you wont need conversions - you can substract your dates as chars (If they look this way '2016-12-31 11:02:00')

Answer 2

Since the column was formatted in nvarchar32 rather than date/time which it should have been, it resulted in periodically the database receiving a text value for (some) open records for the finish time rather than a NULL value.
This occurred only periodically and for whatever reason proposed by whoever designed the database.
This ideally shouldn't happen since its against database convention to do this kind of thing, as NULL values are simply ignored by calculation functions since they are unknown. Still unclear why the case expression didn't resolve the issue, but using the filter FinishTime <> "error text", I resolved the problem.