SELECT Date, DatePart(hour,Date) as Hr, reading
FROM bin1_empty A
WHERE EXISTS( 
(SELECT Min(Date) as date_B
FROM bin1_empty B
WHERE DatePart(hour,A.Date) = DatePart(hour,B.Date) 
HAVING B.Date_B = A.Date)
OR EXISTS(
SELECT Min(Date) as date_C
FROM bin1 C
WHERE DatePart(hour,A.Date) = DatePart(hour,C.Date)
HAVING C.Date_C = A.Date)