The result must be the same for all three expressions.
The question is: which is correct?
The return value for "DATE_ADD(101, INTERVAL 1 hour_second)"
is '2000-01-01 00:00:01'. But the data type in this context is VARCHAR
(note VARCHAR is correct, as the argument is not a temporal data type!).
So it should probably return 2000 in all three cases, i.e. parse
the returned value as a string, without trying to treat it as a datetime.