Querying parent category - sql

Question

I have following query now:
SELECT DISTINCT c.Name as CategoryName, SUM(Amount) OVER(partition by c.Name order by c.Name) as Total
FROM Statements s
LEFT JOIN Categories c on c.Id = s.CategoryId
WHERE Date >= '2016-01-01'
ORDER BY c.Name
It works so far and gets every category with its total amount but what I need is the following.
The categories can have child categories. I want to get the sum over every root category and all its child categories for every row.
Means category A has childs B and C. I want to get the sum over all entries in Statements which have category A, B or C.
The category table consists of following columns: Id, Name, ParentId.
The statements table consists of following coluimns: Id, Date, Amount, CategoryId
Sample data of table category:
Id, Name, ParentId
1, A, null,
2, B, 1
3, C, 2
4, D, null
Sample data of table Statements
Id, Date, Amount, CategoryId
1, 2016-01-01, 100, 1
2, 2016-01-01, 200, 2
3, 2016-01-01, 800, 4
4, 2016-01-01, 300, 3
The output of the query should be as follows:
CategoryName, Total
A, 600
D, 800
Ideally I could pass the parentCategoryId into the where clause which is in my sample null but could also be 2 for instance.
Any hints appreciated :)

Answer 1

A common way is to use a recursive CTE to build the hierarchy..
;
WITH cte
AS (
SELECT [Id],
[Name],
[ParentId],
[Name] AS [Root]
FROM Categories
WHERE ParentId IS NULL
UNION ALL
SELECT c.[Id],
c.[Name],
c.[ParentId],
[Root]
FROM Categories c
JOIN cte ON c.ParentID = cte.Name
)
SELECT cte.[Root] AS [CategoryName],
SUM(Amount) AS [Total]
FROM cte
JOIN Statements s ON s.CategoryId = cte.Id
GROUP BY cte.[Root]

Answer 2

This is not an answer to the question (which as I write this lacks sufficient information about how the parent/child relationship is described in the data). But, the query is just not the right query for the OP's stated purpose. The right query is a simple aggregation:
SELECT c.Name as CategoryName, SUM(Amount) as Total
FROM Table s LEFT JOIN
Categories c
on c.Id = s.CategoryId
WHERE Date >= '2016-01-01'
GROUP BY c.Name
ORDER BY c.Name;
I would be quite surprised if the query in the question actually produced useful results, given the duplicated rows for each category.