Recursive joins - sql

Question

I have two tables defined below (note that the Regions table is recursive and that the recursion can potentially have many levels).
Regions
Id
ParentId
Name
1
null
EU
2
1
Germany
3
1
France
Cities
Id
Name
RegionId
1
Berlin
2
2
Hamburg
2
3
Paris
3
4
Nice
3
I want to see how many cities there are in a particular region. Desired output below:
Region
CityCount
EU
4
Germany
2
France
2
This query gives me the count of cities in every child region, but how do I join in the recursive table to also get the parent (in this case EU) region?
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
I've tried to simplify a real-world problem I'm facing, this is obviously the simplification.

Answer 1

This appears to be what you're after. You can use an rCTE to move through the hierachy to the root, but each iteration retains certain information; in this case the name of the original node. Then you can still JOIN on the 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

Answer 2

You can use a recursive CTE to flatten your regions tree:
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
Then use that CTE in the JOIN of your cities table:
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
See this db<>fiddle.