select hour(ADD_TIME),sum(SL)
from a
where ADD_TIME between '2011-09-24' and '2011-09-25' 
group by hour(ADD_TIME)

------解决方案--------------------
探讨

SQL code
select DATE_FORMAT(ADD_TIME,'%H') as hh,sum(SL) as sl 
from a
where ADD_TIME >='2011-09-24' and ADD_TIME <'2011-09-25' 
group by DATE_FORMAT(ADD_TIME,'%H') 
order by DATE_FORMAT(ADD_TIME……