select top 1 id,datediff( minute,logintime,logouttime) as minutes from UserLoginLog where DATEDIFF(day, logintime, '2011-8-29')=0 and DATEDIFF(day, logouttime, '2011-8-29')=0
logintime,logouttime两个日期字段
两字段间隔分钟数：datediff( minute,logintime,logouttime) 
where条件获得某一天的记录
DATEDIFF(day, logintime, '2011-8-29')=0