SQL NESTED JOIN ISSUES - sql-server

Question

I'm having a heck of a time getting my query to work properly. I have 3 tables (ORDERS, ORDERSITEMS, ORDERPAYMENTS) that I'm trying to build a report for monthly sales taxes due which will look like this:
MONTH YEAR TOTAL RECEIPTS EXEMPT RECEIPTS NON-EXEMPT RECEIPTS SALES TAX
1 2020 $5,000 $4,500 $500 $31.25
TOTAL RECEIPTS: To get this number you have to add together all of the C.OrderPayments_Amount for the given time frame, in this case MONTH(C.OrderPayments_Date) = 1 AND YEAR(C.OrderPayments_Date) = 2020
EXEMPT RECEIPTS: You have to determine if an item is taxable (eg. digital files are not sales taxable, physical prints are). To get this you have determine if the item is taxable by checking the B.OrdersItems_ItemChargeSalesTax. If this field is 0 it does not get charged sales tax and if it is 1 then it does. You then have to filter to only get ORDERSITEMS for the Jan 2020 time frame using by grouping by the C.OrderPayments_Date column that have the B.OrdersItems_ItemChargeSalesTax = 0 and finally add the B.OrdersItems_Total together.
NON-EXEMPT RECEIPTS: To get this number you do the same exact thing for EXEMPT RECEIPTS above except you look for B.OrdersItems_ItemChargeSalesTax = 1 and add the B.OrdersItems_Total together.
SALES TAX: To get this number you use the same date filter as before MONTH(C.OrderPayments_Date) = 1 AND YEAR(C.OrderPayments_Date) = 2020 and then add the A.Order_SalesTax column together.
The first query I have listed is working fine to bring me the total taxes paid and the total income for the month (right now I have hard coded the WHERE statement but that is going to be applied via filters on my list page). What I
need to get from the ORDERSITEMS table is a field called ORDERSITEMS_ItemTotal and sum that field so it is a single line entry on the query. I have seen another person do a nested join. Any suggestions would be greatly appreciated.
**ORDERS TABLE "A"**
Order_ID,
Order_SalesTax,
stateTaxAbbreviation
**ORDERSITEMS TABLE "B"**
Order_FK,
OrdersItems_ItemChargeSalesTax,
OrdersItems_Total
NOTE: In the ORDERSITEMS table a single Order_FK may appear several times as there can be many items on an order
**ORDERPAYMENTS TABLE "C"**
Order_FK,
OrderPayments_PaymentDate,
OrderPayments_Amount
NOTE: In the ORDERPAYMENTS table a single Order_FK may appear several times as there can be multiple payments on an order
While writing this out it seems to be an easy task but when I attempt to put it all together the numbers are wrong because it is adding entries multiple times because there are multiple items on an order and thus it is adding the total payment every time.
Here is the code that I've been tinkering with and I would really appreciate any guidance. Thank you in advance and hopefully I've explained my situation clearly enough.
Select
a.stateTaxAbbreviation AS StateAbbr,
MONTH(c.OrderPayments_Date) AS PaymentMonth,
YEAR(c.OrderPayments_Date) AS PaymentYear,
SUM(c.OrderPayments_Amount) AS TotalPayments,
SUM(a.Order_SalesTax) AS sales_tax
FROM dbo.ORDERS a
INNER JOIN ORDERPAYMENTS as c ON c.Order_FK = a.Order_ID
LEFT OUTER JOIN ORDERITEMS b on b.Order_FK = a.Order_ID
WHERE a.stateTaxAbbreviation = 'MA' AND Month(b.OrderPayments_Date) = 1 AND YEAR(b.OrderPayments_Date) = 2020
GROUP BY stateTaxAbbreviation , MONTH(c.OrderPayments_Date), Year(c.OrderPayments_Date)

Answer 1

You should probably write 2 queries, one where you join the ORDERPAYMENTS table, and another where you join the ORDERITEMS table. Then you can combine them with UNION ALL.
Something like this:
SELECT StateAbbr,PaymentMonth,PaymentYear,SUM(TotalPayments),SUM(sales_tax),SUM(OrdersItems_Total)
FROM (
Select
a.stateTaxAbbreviation AS StateAbbr,
MONTH(c.OrderPayments_Date) AS PaymentMonth,
YEAR(c.OrderPayments_Date) AS PaymentYear,
SUM(c.OrderPayments_Amount) AS TotalPayments,
SUM(a.Order_SalesTax) AS sales_tax,
0 as OrdersItems_Total
FROM dbo.ORDERS a
INNER JOIN ORDERPAYMENTS as c ON c.Order_FK = a.Order_ID
WHERE a.stateTaxAbbreviation = 'MA' AND Month(c.OrderPayments_Date) = 1 AND YEAR(c.OrderPayments_Date) = 2020
GROUP BY stateTaxAbbreviation , MONTH(c.OrderPayments_Date), Year(c.OrderPayments_Date)
UNION ALL
Select
a.stateTaxAbbreviation AS StateAbbr,
MONTH(c.OrderPayments_Date) AS PaymentMonth,
YEAR(c.OrderPayments_Date) AS PaymentYear,
0 AS TotalPayments,
0 AS sales_tax,
SUM(B.OrdersItems_Total) as OrdersItems_Total
FROM dbo.ORDERS a
INNER JOIN ORDERITEMS b on b.Order_FK = a.Order_ID
WHERE a.stateTaxAbbreviation = 'MA' AND Month(b.OrderPayments_Date) = 1 AND YEAR(b.OrderPayments_Date) = 2020
GROUP BY stateTaxAbbreviation , MONTH(c.OrderPayments_Date), Year(c.OrderPayments_Date)
)t
GROUP BY StateAbbr,PaymentMonth,PaymentYear

Answer 2

Thanks to Wouter for pointing me in the right direction. After looking at his suggestion I went back and evaluated what I needed and I create the solution that worked that was based on his idea. Thanks for your patience Wouter, your insight helped a lot!
Select StateAbbr, OrderYear, OrderMonth, SUM(TotalSales) As TotalSales, SUM(TotalSales)-SUM(TaxableRevenue) As ExemptRevenue, SUM(TaxableRevenue) As TaxableRevenue, SUM(SalesTax) As SalesTax
FROM (
Select
/*Get Total Sales and Total Sales Tax Collected*/
a.stateTaxAbbreviation AS StateAbbr,
MONTH(a.Order_Date) As OrderMonth,
YEAR(a.Order_Date) As OrderYear,
SUM((a.Order_TotalBaseSale + a.Order_Shipping) - (((a.Order_PercentDiscount*a.Order_TotalBaseSale)/100) + a.Order_DollarDiscount)) As TotalSales,
SUM(0) As ExemptRevenue,
Sum(0) As TaxableRevenue,
SUM(a.Order_SalesTax) AS SalesTax
FROM dbo.ORDERS a
WHERE a.Order_Status != 'Cancelled'
Group By a.stateTaxAbbreviation, MONTH(a.Order_Date), YEAR(a.Order_Date)
UNION ALL
Select
/*GET EXEMPT ORDERS*/
a.stateTaxAbbreviation AS StateAbbr,
MONTH(a.Order_Date) As OrderMonth,
YEAR(a.Order_Date) As OrderYear,
Sum(0) As TotalSales,
Sum(OrdersItems_ItemTotal) AS ExemptRevenue,
Sum(0) AS TaxableRevenue,
Sum(0) As SalesTax
FROM ORDERSITEMS b
LEFT JOIN ORDERS a ON Order_ID = b.Order_FK
WHERE b.OrdersItems_ItemChargeSalesTax = 0 and a.Order_Status != 'Cancelled'
Group By a.stateTaxAbbreviation, MONTH(a.Order_Date), YEAR(a.Order_Date)
UNION ALL
Select
/*GET NON-EXEMPT ORDERS*/
a.stateTaxAbbreviation AS StateAbbr,
MONTH(a.Order_Date) As OrderMonth,
YEAR(a.Order_Date) As OrderYear,
SUM(0) As TotalSales,
SUM(0) AS ExemptRevenue,
Sum(OrdersItems_ItemTotal) AS TaxableRevenue,
Sum(0) As SalesTax
FROM ORDERSITEMS b
LEFT JOIN ORDERS a ON Order_ID = b.Order_FK
WHERE b.OrdersItems_ItemChargeSalesTax <> 0 and a.Order_Status != 'Cancelled'
Group By a.stateTaxAbbreviation, MONTH(a.Order_Date), YEAR(a.Order_Date)
)t
GROUP BY StateAbbr, OrderMonth, OrderYear
ORDER BY StateAbbr ASC, OrderYear DESC, OrderMonth ASC