SQL 時間相減

DATEDIFF ( datepart , startdate , enddate )

datepart	Abbreviations
year	yy,yyyy
quater	qq, q
month	mm, m
dayofyear	dy, y
day	dd, d
week	wk, ww
hour	hh
minute	mi, n
second	ss, s
millisecond	ms
microsecond	mcs
nanosecond	ns