在SQL服务器上显示一个视图，以显示父级和子级的产品。

Question

我有一个这样的表格。

+----+----------+----------+-----------+
| ID |   Name   | Is_Group | Parent_id |
+----+----------+----------+-----------+  
|  1 | Clothes  |        1 | Null      |  
|  2 | Food     |        1 | Null      |  
|  3 | fastfood |        1 | 2         |  
|  4 | T-shirt  |        0 | 1         |
|  5 | skirt    |        0 | 1         |
|  6 | pizza    |        0 | 3         |
|  7 | snack    |        0 | 3         |
+----+----------+----------+-----------+

我想用横向的方式来做报告，比如：

+----+---------+---------+----------+
| ID |  Name   | level1  |  level2  |
+----+---------+---------+----------+
|  4 | T-shirt | Clothes | Null     |
|  5 | skirt   | Clothes | Null     |
|  6 | pizza   | Food    | fastfood |
|  7 | snack   | Food    | fastfood |
+----+---------+---------+----------+

有谁知道怎么做？

Answer 1

你可以用两级的 left join:

select t.*,
       coalesce(tpp.name, tp.name) as level1,
       (case when tpp.name is not null then tp.name end) as level2
from t left join
     t tp
     on t.parent_id = tp.id left join
     t tpp
     on tp.parent_id = tpp.parent_id
where not exists (select 1
                  from t tc
                  where tc.parent_id = t.id);