private void tree(List<Article> articles, Connection conn, int id, int grade){String sql = "select * from article where pid =" +id;Statement stmt = BBSDB.createStmt(conn);ResultSet rs = BBSDB.executeQuery(stmt, sql);BBSDB.close(stmt);//BBSDB.close(conn);create table `bbs`.`article`( `id` int not null auto_increment, `pid` int, `rootid` int, `title` varchar(255), `cont` text, `pdate` datetime, `isleaf` int, primary key (`id`) );