mysql使用PreparedStatement的一个奇怪问题？？？
public   boolean   IsExistStudent(String   username)throws   SQLException{
      boolean   flag=false;
      String   sql= "select   *   from   user   where   username=? ";
      Connection   conn=DBconnect.getConnection();
      PreparedStatement   pstat=null;
      pstat=conn.prepareStatement(sql);
      pstat.setString(1,username);
      ResultSet   rs=null;
      rs=pstat.executeQuery(sql);
      if(rs.next()){
      flag=true;
      }
      DBconnect.close(rs,   pstat,   conn);
      return   flag;
   
     
      }


使用的是mysql5.0     错误提示：You   have   an   error   in   your   SQL   syntax;   check   the   manual   that   corresponds   to   your   MySQL   server   version   for   the   right   syntax   to   use   near   '? '   at   line   1StudentControl   Servlet   error:You   have   an   error   in   your   SQL   syntax;   check   the   manual   that   corresponds   to   your   MySQL   server   version   for   the   right   syntax   to   use   near   '? '   at   line   1StudentControl   Servlet   error:You   have   an   error   in   your   SQL   syntax;   check   the   manual   that   corresponds   to   your   MySQL   server   version   for   the   right   syntax   to   use   near   '? '   at   line   1


可是这个方法却能正常运行
public   void   addStudent(Student   s)throws   SQLException{
  String   sql= "insert   into   user   values(?,?,?,?,?,?) ";
  Connection   conn=DBconnect.getConnection();
  PreparedStatement   pstat=conn.prepareStatement(sql);
  pstat.setString(1,   s.getUsername());
  pstat.setString(2,   s.getPassword());
  pstat.setString(3,   s.getEmail());
  pstat.setInt(4,   s.getAge());
  pstat.setString(5,   s.getClasses());
  pstat.setString(6,   s.getSex());
  pstat.executeUpdate();
  DBconnect.close(null,   pstat,   conn);
 
  }

大家帮忙看看怎么回事。

------解决方案--------------------
难道你用的是全角的问号？