递归连接

Question

我在下面定义了两个 table（请注意 Regions table 是递归的，并且递归可能有很多级别）。

地区

Id	ParentId	Name
1	null	EU
2	1	Germany
3	1	France

城市

Id	Name	RegionId
1	Berlin	2
2	Hamburg	2
3	Paris	3
4	Nice	3

我想看看某个地区有多少个城市。所需输出如下：

Region	CityCount
EU	4
Germany	2
France	2

此查询为我提供了每个子区域中的城市数量，但我如何加入递归 table 以获得父区域（在本例中为 EU）区域？

select R.Name, count(C.Id)
from Regions R
join Cities C on C.RegionId = R.Id
group by R.Name
having count(C.Id) > 1

我试图简化我面临的现实问题，这显然是简化。

Answer 1

您可以使用递归 CTE 来展平您的 regions 树：

with flatregions as
(
  select t.ID, t.ParentID, t.Name, 1 as lvl
  from regions t

  union all

  select t.ID, tt.ParentID, tt.Name, t.lvl+1
  from flatregions t
  inner join regions tt on tt.ID = t.ParentID
)
select * FROM flatregions;

ID	ParentID	Name	lvl
1		EU	1
2	1	Germany	1
3	1	France	1
3		EU	2
2		EU	2

然后在 cities table 的 JOIN 中使用该 CTE：

with flatregions as
(
  select t.ID, t.ParentID, t.Name, 1 as lvl
  from regions t

  union all

  select t.ID, tt.ParentID, tt.Name, t.lvl+1
  from flatregions t
  inner join regions tt on tt.ID = t.ParentID
)
select R.Name, count(C.Id) as CityCount
from flatregions R
join Cities C on C.RegionId = R.Id
group by R.Name
having count(C.Id) > 1

Region	CityCount
EU	4
Germany	2
France	2

参见 this db<>fiddle。

Answer 2

这似乎是您想要的。您可以使用 rCTE 通过层次结构移动到根，但每次迭代都会保留某些信息；在这种情况下，原始节点的名称。然后你仍然可以 JOIN 在 RegionID:

WITH rCTE AS(
    SELECT R.ID,
           R.ParentID,
           R.[Name]
    FROM dbo.Regions R
    UNION ALL
    SELECT R.ID,
           R.ParentID,
           C.[Name]
    FROM dbo.Regions R
         JOIN rCTE C ON R.ParentID = C.ID)
SELECT r.[Name],
       COUNT(*) AS CityCount
FROM rCTE r
     JOIN dbo.Cities C ON r.ID = C.RegionID
GROUP BY r.[Name];

db<>fiddle

递归连接

Recursive joins

sql

sql-server

recursive-query