高级查询时出现下列错误如何解决？

//if($_POST[项目名称] != null)
    $sqlstr = $sqlstr."项目名称 like '%".$_POST[项目名称]."%' ";
if($_POST[s_time] <= $_POST[e_time])
    $sqlstr = $sqlstr." and (立项日期 between '".$_POST[s_time]."-01-01' and '".$_POST[e_time]."-12-31')";
else
    $sqlstr = $sqlstr." and (立项日期 between '".$_POST[e_time]."' and '".$_POST[s_time]."')";
   
    $sqlstr = $sqlstr."and 主研单位 like '%".$_POST[主研单位]."%' ";
    $result = mysql_query($sqlstr,$conn);

错误提示：Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in E:\E\AppServ\www\edit\index.php on line 334

line 334：hile ($myrow=mysql_fetch_array($result)){
请高手解答，谢谢！