update guestbook set today_count =
(case when datediff(day,today_time,getdate()) <> 0 then 0 end),
today_time = getdate()
where id = @Userid
(case when datediff(day,today_time,getdate()) <> 0 then 0 end),
today_time = getdate()
where id = @Userid
where today_time<>convert(char(8),getdate(),111)
/*日期格式 yyyy/mm/dd */