your tables and sample data
create table #Category(Id int identity(1,1),Name Varchar(255),parentId int)
INSERT INTO #Category(Name,parentId) values
('Category1',null),('Category1.1',1),('Category2.1',2),
('Category1.2',1),('Category3.1',3)
create table #CategoryItem(Id int identity(1,1),categoryId int,itemId int)
INSERT INTO #CategoryItem(categoryId,itemId) values
(5,1),(4,2),(5,2),(3,1),(2,3),(1,1),(3,2)
create table #Item(Id int identity(1,1),Name varchar(255))
INSERT INTO #Item(Name) values('item1'),('item2'),('item3')
Checking for all childs of parent by Recursive Commom Table Expressions
;WITH CategorySearch(ID, parentId) AS
(
SELECT ID, ID AS ParentId FROM #Category
UNION ALL
SELECT CT.Id,CS.parentId FROM #Category CT
INNER JOIN CategorySearch CS ON CT.ParentId = CS.ID
)
select * from CategorySearch order by 1,2
Output: All child records against parent
ID parentId
1 1
2 1
3 1
4 1
5 1
2 2
3 2
5 2
3 3
5 3
4 4
5 5
Final query for your result, count all items for category and its children categories.
;WITH CategorySearch(ID, parentId) AS
(
SELECT ID, ID AS ParentId FROM #Category
UNION ALL
SELECT CT.Id,CS.parentId FROM #Category CT
INNER JOIN CategorySearch CS ON CT.ParentId = CS.ID
)
SELECT CA.Name AS CategoryName,count(itemId) CountItem
FROM #Category CA
INNER JOIN CategorySearch CS ON CS.ParentId = CA.id
INNER JOIN #CategoryItem MI ON MI.CategoryId =CS.ID
GROUP BY CA.Name
Output:
CategoryName CountItem
Category1 7
Category1.1 5
Category1.2 1
Category2.1 4
Category3.1 2