Don't have a lot of time, but this should be real close at least:

SELECT *,
  CASE WHEN EndDayNumber >= StartDayNumber THEN EndDayNumber - StartDayNumber
   ELSE...

I don't understand why you'd say that.  The main reason I could see for continuing to do backups of...

As noted, the datetime is stored in a numeric format.

It's likely that whatever interface is displaying the date to you is putting it in this format:
Apr 19...

SELECT *,
    CASE WHEN string2 LIKE '%[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]%'
         THEN SUBSTRING(string2, PATINDEX('%[0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]%', string2), 10)
         ELSE NULL END AS...

Don't use functions on any column unless you have absolutely no choice. In other words, keep the condition sargable if at all possible

--If you...

You could make the restriction 100% or less.  The main thing is not overfill it, right?  That would allow changes, you would just have to remove/reduce something before adding something...


SELECT *
FROM PeopleDetail PD
WHERE PD.Name IN (
    SELECT Name
    FROM People
)

Standard CROSS JOIN:

SELECT StudentName, RoomName
FROM Students
CROSS JOIN ( VALUES('room1'),('room2'),('room3') ) AS Rooms(RoomName)