计算两个时间差值的函数，返回时间差的绝对值：   
private   string   DateDiff(DateTime   DateTime1,DateTime   DateTime2)   
{   
   string   dateDiff=null;   
   try   
   {   
     TimeSpan   ts1=new     TimeSpan(DateTime1.Ticks);   
     TimeSpan   ts2=new     TimeSpan(DateTime2.Ticks);   
     TimeSpan   ts=ts1.Subtract(ts2).Duration();   
     dateDiff=ts.Days.ToString()+"天"   
         +ts.Hours.ToString()+"小时"   
         +ts.Minutes.ToString()+"分钟"   
         +ts.Seconds.ToString()+"秒";   
   }   
   catch   
   { 

   }   
   return   dateDiff;   
}

------解决方案--------------------
sqlstr="select * from tablename where datediff(d,时间字段,getdate())<=3"