Database

FileTables (SQL Server)[^]:

In other words, you can store files and documents in special tables in SQL Server called FileTables, but access them from Windows applications as if they were stored in the file system, without making any changes to your client applications.

, Datediff(SECOND,
        convert(datetime, [START_DATE] + ' ' + [START_TIME]),
        convert(datetime, isnull([END_DATE],[START_DATE]) + ' ' + isnull([END_TIME],[START_TIME])))                                               AS M_Time_Worked

declare @t table(START_DATE varchar(10), END_TIME_1 varchar(8), START_TIME_1 varchar(8), END_DATE varchar (10))
insert @t values('', '20:12:52', '', '01/18/2015')

select Datediff(SECOND,
             convert(datetime, [START_DATE] + ' ' + [START_TIME_1]),
             convert(datetime, [END_DATE] + ' ' + [END_TIME_1]))
from @t

declare @t table(START_DATE DateTime, END_DATE DateTime)
insert @t values ('01-jan-2015 20:12:52', Null)

select Datediff(SECOND,[START_DATE], ISNULL([END_DATE],[START_DATE]))
from @t

, CAST(Replace(convert(date, [START_DATE], 101), '-', '') as numeric(8,0))                              AS [START_DATE]

, CAST(dbo.LPAD(dbo.CONVERT_TIME_TO_NUMERIC(SUBSTRING(dbo.arKD.START_TIME,1,8)), 6, 0)AS nvarchar(6))   AS START_TIME

, CAST(Replace(convert(date, END_DATE, 101), '-', '') as numeric(8,0))                                  AS END_DATE

, CAST(dbo.LPAD(dbo.CONVERT_TIME_TO_NUMERIC(SUBSTRING(dbo.arKD.END_TIME,1,8)), 6, 0)AS nvarchar(6))     AS END_TIME

--DATEDIFF(interval, Starting_date datetime, ending_date datetime)
,Datediff(SECOND,
     convert(datetime, [START_DATE] + ' ' + [START_TIME]),
     convert(datetime, [END_DATE] + ' ' + [END_TIME]))                                                  AS M_Time_Worked

,Datediff(SECOND,
     convert(datetime, [START_DATE] + ' ' + [START_TIME]),
     convert(datetime, CASE WHEN RTRIM([END_DATE]) = '' THEN [START_DATE] ELSE [END_DATE] END + ' ' + CASE WHEN RTRIM([END_TIME]) = '' THEN [START_TIME] ELSE [END_TIME] END))                                                  AS M_Time_Worked

The datediff function resulted in an overflow. The number of dateparts separating two date/time instances is too large. Try to use datediff with a less precise datepart.

Ambertje wrote:

I need my dates as string because I'm working with a Data warehouse DB.

,Datediff(SECOND,
     convert(datetime, [START_DATE] + ' ' + [START_TIME]),
     convert(datetime, [END_DATE] + ' ' +   END_TIME]))                                                  AS M_Time_Worked

Ambertje wrote:

So I think ISNULL(Start_Date, Start_Time) is what I need.

ISNULL(Start_Date, EndDate) + ' ' + ISNULL(Start_Time, Endtime)

,Datediff(SECOND,
     convert(datetime, ISNULL( NULLIF([START_DATE], '' ) , END_DATE) + ' ' + ISNULL( NULLIF([START_TIME], '' ) , END_TIME)),
     convert(datetime, [END_DATE] + ' ' + [END_TIME]))                                                  AS M_Time_Worked

, CASE WHEN [START_DATE] IS NULL OR [START_DATE] = '' THEN
    CAST(Replace(convert(date, END_DATE, 101), '-', '') as numeric(8,0))    ELSE
        CAST(Replace(convert(date, [START_DATE], 101), '-', '') as numeric(8,0)) END                    AS [START_DATE]