Assistance on simple query

No idea if this is correct, but it may get you started.

Create some tables with test data.

DROP TABLE IF EXISTS dbo.ORDR;	--Customer Orders
DROP TABLE IF EXISTS dbo.RDR1;	--Lines on each order
GO
CREATE TABLE dbo.ORDR(
	DocEntry	int	NOT NULL PRIMARY KEY,
	DocNum		int	NOT NULL,
	OrderDate	Date	NOT NULL
)

CREATE TABLE dbo.RDR1(
	id		int	IDENTITY(1, 1)	NOT NULL PRIMARY KEY,
	DocEntry	int	NOT NULL,
	PriceBefDi	float	NOT NULL,
	Price		float	NOT NULL,	
	Quantity	int	NOT NULL
)
GO
INSERT INTO dbo.ORDR (DocEntry, DocNum, OrderDate) VALUES
(1, 100, '20200201'),
(2, 101, '20200202')

INSERT INTO dbo.RDR1(DocEntry, PriceBefDi, Price, Quantity) VALUES
(1, 100.0, 98.35, 12),
(1, 50.0, 50.0, 120),
(1, 1000.0, 20.0, 1)
--no order lines for order 2
GO

This is the query to get the total discount for all the orders with an orderDate in the current year.

;WITH CTE AS (
SELECT ORDR.DocEntry
	, SUM(Price * Quantity) as OrderPrice
	, SUM((PriceBefDi - Price) * Quantity) AS OrderDiscount
FROM dbo.ORDR as ORDR
	INNER JOIN dbo.RDR1 as RDR1
		ON ORDR.DocEntry = RDR1.DocEntry
WHERE YEAR(ORDR.OrderDate) = 2020
GROUP BY ORDR.DocEntry
)
SELECT ORDR.DocEntry
	, ORDR.DocNum
	, ORDR.OrderDate
	, COALESCE(CTE.OrderPrice, 0.0) as OrderPrice
	, COALESCE(CTE.OrderDiscount, 0.0) as OrderDiscount
FROM dbo.ORDR as ORDR
	LEFT OUTER JOIN CTE
		ON ORDR.DocEntry = CTE.DocEntry