查询文章的最后回复时间

表：id,title,postuser,postdate,parentid
                  准备sql语句：
                  drop table if exists articles;
                  create table articles(id int auto_increment primary key,title
                  varchar(50), postuser varchar(10), postdate datetime,parentid
                  int references articles(id));
                  insert into articles values
                  (null,'第一条','张三','1998-10-10 12:32:32',null),
                  (null,'第二条','张三','1998-10-10 12:34:32',null),
                  (null,'第一条回复1','李四','1998-10-10 12:35:32',1),
                  (null,'第二条回复1','李四','1998-10-10 12:36:32',2),
                  (null,'第一条回复2','王五','1998-10-10 12:37:32',1),
                  (null,'第一条回复3','李四','1998-10-10 12:38:32',1),
                  (null,'第二条回复2','李四','1998-10-10 12:39:32',2),
                  (null,'第一条回复4','王五','1998-10-10 12:39:40',1);
                  答案：
                  select a.title,a.postuser,(select max(postdate) from articles where parentid=a.id) reply from articles a where a.parentid is null;