I've recently been tasked with migrating some code stacks for an applicatin that is moving from using Access to using SQL-Server.

I don't usually use Pivot all that much, and most of my code is scripting intended for in-house administration of our SQL Servers. As such my code is not that great.

USE [Database1]
GO
SELECT
       Region,
       Year,
       Month,
       SalesPersonNR,
       (ISNULL(SUM(SalesA),0) + ISNULL(SUM(SalesB),0) + ISNULL(SUM(SalesC),0) + ISNULL(SUM(Sales),0) + ISNULL(SUM(NA),0)) AS Total,
       SUM(Sales) Sales,
       SUM(SalesA) SalesA,
       SUM(SalesB) SalesB,
       SUM(SalesC) SalesC
FROM
(
SELECT 
       Region,
       Year,
       Month,
       SalesPersonNR,
       COUNT(SalesNumber) as total,
       expr1
FROM (
       SELECT A.Region,
       Year([Date]) AS Year,
       Month([Date]) AS Month,
       A.SalesPersonNR,
       A.SalesNumber AS SalesNumber,
       CASE 
             WHEN B.DateSalesC IS NOT NULL THEN 'SalesC'
             WHEN B.DateSalesB IS NOT NULL THEN 'SalesB'
             WHEN B.DateSalesA IS NOT NULL THEN 'SalesA'
             WHEN B.DateSales IS NOT NULL THEN 'Sales'
             ELSE 'NA'
       END AS expr1
FROM [Table1] A 
       JOIN [Table2] B 
       ON A.SalesNumber = B.SalesNumber 
WHERE 
       A.Status = 99 
       ) Internal
GROUP BY 
       Region,
       Year,
       Month,
       SalesPersonNR,
       expr1
) AS x
PIVOT
(
       SUM(Total)
       FOR expr1 IN (SalesC, SalesB, SalesA, Sales, NA)
       ) AS y
GROUP BY 
Region,
Year,
Month,
SalesPersonNR

How could I clean up this code, and make it easier to maintain.
My main concern is, I want this simpler, but how?