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?