假设我有一个这样的 article_category 树：
GrandPa Cate
|______ Parent Cate 1 (article 1, article2)
|______ Parent Cate 2
|______|_____ Child Cate 1 (article 3, article 4)
|______|_____ Child Cate 2 (article 5)
表：
类别
id | parent_id | title
----------------------------------------
1 | null | GrandPa Cate
----------------------------------------
2 | 1 | Parent Cate 1
----------------------------------------
3 | 1 | Parent Cate 2
----------------------------------------
4 | 3 | Child Cate 1
----------------------------------------
5 | 3 | Child Cate 2
----------------------------------------
文章
id | cate_id | title
----------------------------------------
1 | 2 | article 1
----------------------------------------
2 | 2 | article 2
----------------------------------------
3 | 4 | article 3
----------------------------------------
4 | 4 | article 4
----------------------------------------
5 | 5 | article 5
----------------------------------------
选择Granpa Cate如何获得全部5篇文章？