====== Sum datetime difference in MSSQL ======
Get the SUM of seconds:

<code>SET @Seconds = SELECT SUM(DATEDIFF(SECOND, [START_TIME], [END_TIME]))</code>

Turn that into a datetime object:

<code>SELECT CONVERT(VARCHAR(8), dateadd(SECOND, @Seconds, 0), 108)</code>

Or as 1 query:

<code>SELECT CONVERT(VARCHAR(8), dateadd(SECOND, SUM(DATEDIFF(SECOND, [START_TIME], [END_TIME])), 0), 108)</code>