Percentage difference from previous year - sql

Question

Trying to add a percent difference column to my data as i'm showing the last 3 years of sales and commissions in my query and want percent of difference for each salesperson listed to show the difference in the amounts that were made for sales and also for commission from the previous year in that same month. So for the amount made in January 2017 for commission and sales, I want to show whether they had an increase or decrease in the amount between what was earned in January 2018 compared to what was earned in January 2017.
SELECT TOP (100) PERCENT
'ABC' AS CompanyCode,
ABC.AR_Salesperson.SalespersonName,
ABC.AR_SalespersonCommission.SalespersonDivisionNo,
ABC.AR_SalespersonCommission.SalespersonNo,
SUM(ABC.AR_SalespersonCommission.InvoiceTotal) AS InvoiceTotalSum,
SUM(ABC.AR_SalespersonCommission.CommissionAmt) AS CommissionAmtSum,
DATENAME(month, ABC.AR_SalespersonCommission.InvoiceDate) AS Month,
DATENAME(year, ABC.AR_SalespersonCommission.InvoiceDate) AS Year,
DATEPART(m, ABC.AR_SalespersonCommission.InvoiceDate) AS MonthNumber
FROM
ABC.AR_Customer INNER JOIN
ABC.AR_SalespersonCommission ON
ABC.AR_Customer.ARDivisionNo = ABC.AR_SalespersonCommission.ARDivisionNo AND
ABC.AR_Customer.CustomerNo = ABC.AR_SalespersonCommission.CustomerNo INNER JOIN
ABC.AR_Salesperson ON
ABC.AR_SalespersonCommission.SalespersonDivisionNo = ABC.AR_Salesperson.SalespersonDivisionNo AND
ABC.AR_SalespersonCommission.SalespersonNo = ABC.AR_Salesperson.SalespersonNo
GROUP BY
ABC.AR_Salesperson.SalespersonName,
ABC.AR_SalespersonCommission.SalespersonDivisionNo,
ABC.AR_SalespersonCommission.SalespersonNo,
DATENAME(month, ABC.AR_SalespersonCommission.InvoiceDate),
DATENAME(year, ABC.AR_SalespersonCommission.InvoiceDate),
DATEPART(m, ABC.AR_SalespersonCommission.InvoiceDate)
HAVING
(DATENAME(year, ABC.AR_SalespersonCommission.InvoiceDate) > DATEADD(year, - 4, GETDATE()))
ORDER BY
SalespersonName,
Year,
MonthNumber
Tried various ways to get the data to do this but haven't been able to, like using OVER PARTITION BY and all that. Desired Results and Sample Data are in the link below.

Answer 1

You can use a CTE and JOIN to it using the previous year:
; WITH L4Y
AS (SELECT TOP (100) PERCENT
'ABC' AS CompanyCode,
ABC.AR_Salesperson.SalespersonName,
ABC.AR_SalespersonCommission.SalespersonDivisionNo,
ABC.AR_SalespersonCommission.SalespersonNo,
SUM(ABC.AR_SalespersonCommission.InvoiceTotal) AS InvoiceTotalSum,
SUM(ABC.AR_SalespersonCommission.CommissionAmt) AS CommissionAmtSum,
DATENAME(month, ABC.AR_SalespersonCommission.InvoiceDate) AS Month,
DATENAME(year, ABC.AR_SalespersonCommission.InvoiceDate) AS Year,
DATEPART(m, ABC.AR_SalespersonCommission.InvoiceDate) AS MonthNumber
FROM
ABC.AR_Customer INNER JOIN
ABC.AR_SalespersonCommission ON
ABC.AR_Customer.ARDivisionNo = ABC.AR_SalespersonCommission.ARDivisionNo AND
ABC.AR_Customer.CustomerNo = ABC.AR_SalespersonCommission.CustomerNo INNER JOIN
ABC.AR_Salesperson ON
ABC.AR_SalespersonCommission.SalespersonDivisionNo = ABC.AR_Salesperson.SalespersonDivisionNo AND
ABC.AR_SalespersonCommission.SalespersonNo = ABC.AR_Salesperson.SalespersonNo
GROUP BY
ABC.AR_Salesperson.SalespersonName,
ABC.AR_SalespersonCommission.SalespersonDivisionNo,
ABC.AR_SalespersonCommission.SalespersonNo,
DATENAME(month, ABC.AR_SalespersonCommission.InvoiceDate),
DATENAME(year, ABC.AR_SalespersonCommission.InvoiceDate),
DATEPART(m, ABC.AR_SalespersonCommission.InvoiceDate)
HAVING
(DATENAME(year, ABC.AR_SalespersonCommission.InvoiceDate) > DATEADD(year, - 4, GETDATE()))
)
SELECT L4Y.*
, InvoiceTotalPrevYear = COALESCE(LY.InvoiceTotalSum, 0)
, CommissionAmtSumPrevYear = COALESCE(LY.CommissionAmtSum, 0)
FROM L4Y
LEFT JOIN L4Y LY
ON L4Y.CompanyCode = LY.CompanyCode
AND L4Y.SalespersonName = LY.SalespersonName
--...(join on ALL fields except YEAR)
AND l4y.[Month] = LY.[Month]
-- Here's where the magic happens:
AND L4Y.[Year] = (LY.[Year]-1) ;