How to capture results when datestamp has special exception - sql-server

Question

I'm fairly certain I can use a CASE statement in this instance, but I'm clearly not looking at this right.
In my WHERE clause, when a datestamp resides in the month of July, I need to group lump it into my results for the month of August. Here's what I'm trying, but
I'm missing the mark:
and CPTrn_DateTime = case
when datepart(month, CPTrn_DateTime) = 7 then datepart(month, CPTrn_DateTime) in (7,8)
The reason I was trying to use a CASE statement is because only for the month of July do I need those results to be associated with August results. The reason I can't do a simple date range is because I need the logic to work for all dates moving forward as time goes on.
Here's the entire query:
DECLARE #month INT
SET #month = 8
SELECT DISTINCT cp.CPTrn_Key
,ch.Chg_Ref_No
,c.Cust_Alias
,ch.Chg_Total_Units
,ch.Chg_Amount
,cp.CPTrn_DateTime
FROM PDICompany_2049_01..CP_Transactions cp(NOLOCK)
INNER JOIN PDICompany_2049_01..Customers c(NOLOCK) ON CPTrn_Cust_Key = Cust_Key
INNER JOIN PDICompany_2049_01..Products p(NOLOCK) ON cp.CPTrn_Prod_Key = Prod_Key
LEFT JOIN PDICompany_2049_01..CP_Billing_Details CPBD(NOLOCK) ON CPBd.CPBillDtl_CPTrn_Key = cp.CPTrn_Key
AND CPTrn_Cust_Key = CPBD.CPBillDtl_Cust_Key
AND CPBD.CPBillDtl_Rec_Type = 1
LEFT JOIN PDICompany_2049_01..Customer_Locations cl(NOLOCK) ON c.Cust_WhPrcNtc_Def_CustLoc_Key = cl.CustLoc_Key
AND ((CustLoc_Type & 2) <> 0)
LEFT JOIN Charges ch ON ch.Chg_Ref_No = cpbd.CPBillDtl_Invoice_No
WHERE cp.CPTrn_Tran_Type != 0
AND c.Cust_Alias = 'MONTGOMERY CRANES, LLC'
AND CPBillDtl_Invoice_No IS NOT NULL
AND CPTrn_DateTime = CASE
WHEN datepart(month, CPTrn_DateTime) = 7
THEN datepart(month, CPTrn_DateTime) IN (7,8)
ELSE datepart(month, CPTrn_DateTime) = #month
END AS 'CPTrn_DateTime'
ORDER BY 1

Answer 1

Doing this is a CASE expression (Case statements don't exist in T-SQL) with the DATEPART function is going to likely ruin performance.
You don't "group" in a WHERE, you do that in your GROUP By, so i'm reading through the lines, but what's wrong with "normal" date logic?
WHERE CPTrn_DateTime >= '20190701'
AND CPTrn_DateTime < '20190901'
Edit: Simple date logic is still what you are after:
DECLARE #Month int = 8,
#Year int = 2019;
DECLARE #DateStart date = DATEFROMPARTS(#Year, IIF(#Month = 8, 7, #MOnth),1);
DECLARE #DateEnd date = DATEFROMPARTS(#Year, #Month+1, 1);
SELECT ...
FROM...
WHERE CPTrn_DateTime >= #DateStart
AND CPTrn_DateTime < #DateEnd

Answer 2

Or you can do it all in the query:
SELECT DISTINCT cp.CPTrn_Key
,ch.Chg_Ref_No
,c.Cust_Alias
,ch.Chg_Total_Units
,ch.Chg_Amount
,cp.CPTrn_DateTime
FROM PDICompany_2049_01..CP_Transactions cp(NOLOCK)
INNER JOIN PDICompany_2049_01..Customers c(NOLOCK) ON CPTrn_Cust_Key = Cust_Key
INNER JOIN PDICompany_2049_01..Products p(NOLOCK) ON cp.CPTrn_Prod_Key = Prod_Key
LEFT JOIN PDICompany_2049_01..CP_Billing_Details CPBD(NOLOCK) ON CPBd.CPBillDtl_CPTrn_Key = cp.CPTrn_Key
AND CPTrn_Cust_Key = CPBD.CPBillDtl_Cust_Key
AND CPBD.CPBillDtl_Rec_Type = 1
LEFT JOIN PDICompany_2049_01..Customer_Locations cl(NOLOCK) ON c.Cust_WhPrcNtc_Def_CustLoc_Key = cl.CustLoc_Key
AND ((CustLoc_Type & 2) <> 0)
LEFT JOIN Charges ch ON ch.Chg_Ref_No = cpbd.CPBillDtl_Invoice_No
WHERE cp.CPTrn_Tran_Type != 0
AND c.Cust_Alias = 'MONTGOMERY CRANES, LLC'
AND CPBillDtl_Invoice_No IS NOT NULL
AND #month = case when month(CPTrn_DateTime) = 7 then 8 else month(CPTrn_DateTime) end
I believe order by 1 is superfluous.