Try this
SELECT
CD.FirstName+''+CD.MiddleName+' '+CD.LastName as Name
,CD.MobileNumber
,CD.email
,CD.RegisterDate
,SUM(BD.DC_Bill)
FROM Customer_Details CD LEFT JOIN Bill_Details BD ON CD.CustomerID = BD.CustomerID
WHERE BD.Billdate Between @StartDate AND @EndDate
GROUP BY CustomerID
Hope this helps if yes then accept and vote the answer
--Rahul D.