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显示文章标题，发帖人、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);

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