SQL Server 2014 Consolidate Tables avoiding duplicates - sql-server

Question

I have 36 Sales tables each referred to one store:
st1.dbo.Sales
st2.dbo.Sales
...
st35.dbo.Sales
st36.dbo.Sales
Each record has the following key columns:
UserName, PostalCode, Location, Country, InvoiceAmount, ItemsCount, StoreID
Here is SQLFiddle
I need to copy into Customers table all Username (and their details) that are not already present into Customers
in case of duplicated it is required to use the fields of record where InvoiceAmount is MAX
I tried to build a query but looks too complicated and it is also wrong because in CROSS APPLY should consider the full list of Sales Tables
INSERT INTO Customers (.....)
SELECT distinct
d.UserName,
w.postalCode,
w.location,
W.country,
max(w.invoiceamount) invoiceamount,
max(w.itemscount) itemscount,
w.storeID
FROM
(SELECT * FROM st1.dbo.Sales
UNION
SELECT * FROM st2.dbo.Sales
UNION
...
SELECT * FROM st36.dbo.Sales) d
LEFT JOIN
G.dbo.Customers s ON d.Username = s.UserName
CROSS APPLY
(SELECT TOP (1) *
FROM s.dbo.[Sales]
WHERE d.Username=w.Username
ORDER BY InvoiceAmount DESC) w
WHERE
s.UserName IS NULL
AND d.username IS NOT NULL
GROUP BY
d.UserName, w.postalCode, w.location,
w.country, w.storeID
Can somebody please give some hints?

Answer 1

As a basic SQL query, I'd create a row_number in the inner subquery and then join to customers and then isolated the max invoice number for each customer not in the customer table.
INSERT INTO Customers (.....)
SELECT w.UserName,
w.postalCode,
w.location,
w.country,
w.invoiceamount,
w.itemscount,
w.storeID
FROM (select d.*,
row_number() over(partition by d.Username order by d.invoiceamount desc) rownumber
from (SELECT *
FROM st1.dbo.Sales
UNION
SELECT *
FROM st2.dbo.Sales
UNION
...
SELECT *
FROM st36.dbo.Sales
) d
LEFT JOIN G.dbo.Customers s
ON d.Username = s.UserName
WHERE s.UserName IS NULL
AND d.username IS NOT NULL
) w
where w.rownumber = 1

Answer 2

Using your fiddle this will select distinct usernames rows with max invoiceamount
with d as(
SELECT * FROM Sales
UNION
SELECT * FROM Sales2
)
select *
from ( select *,
rn = row_number() over(partition by Username order by invoiceamount desc)
from d) dd
where rn=1;

Answer 3

step 1 - use cte .
select username , invoiceamount ,itemscount from Sales
UNION all
select user name , invoiceamount ,itemscount from Sales
.....
...
step 2
next cte use group by and get max invoiceamount ,itemscount for user of last result set.
,cte2 as (
select user name , max (invoiceamount) as invoiceamount ,max(itemscount) as itemscount from cte)
step3
use left join with user table and find missing record and itemscount invoiceamount