查询为每个级别的父级生成新列答案

【问题标题】：Query to Generate New Column for Parent Each Level查询为每个级别的父级生成新列
【发布时间】：2021-08-04 04:58:43
【问题描述】：

我有一个包含分层数据的简单表：

+----+----------+--------+-------+
| id | name     | parent | level |
+----+----------+--------+-------+
| 1  | province | Null   | 1     |
+----+----------+--------+-------+
| 2  | city     | 1      | 2     |
+----+----------+--------+-------+
| 3  | fire     | 2      | 3     |
+----+----------+--------+-------+
| 4  | ems      | 2      | 3     |
+----+----------+--------+-------+

我正在尝试构建一个递归查询来为每个级别添加一个新列。像这样：

+----+----------+---------+---------+
| id | level_1  | level_2 | level_3 |
+----+----------+---------+---------+
| 1  | Province | Null    | Null    |
+----+----------+---------+---------+
| 2  | Province | City    | Null    |
+----+----------+---------+---------+
| 3  | Province | City    | Fire    |
+----+----------+---------+---------+
| 4  | Province | City    | EMS     |
+----+----------+---------+---------+

我不知道如何解决这个问题，我对 CTE 查询不是很好。有什么建议吗？

【问题讨论】：

标签： sql sql-server tsql common-table-expression

【解决方案1】：

嗯，这很复杂。我正在考虑将union all 与每个级别的单独查询一起使用：

select t1.id, t1.name as level_1, null as level_2, null as level_3
from t t1 
where level = 1
union all
select t2.id, t1.name, t2.name, null
from t t1 join
     t t2
     on t2.parent = t1.id
where t2.level = 2
union all
select t3.id, t1.name, t2.name, t3.name
from t t1 join
     t t2
     on t2.parent = t1.id join
     t t3
     on t3.parent = t2.id
where t3.level = 3;

【讨论】：