Self Join Issue - Defining Extracted Time Periods - sql

Question

In the below query, I extract the month in the Select query. I then execute multiple Self Joins using the extracted month. I am getting an
Invalid column name 'mnth'
for every time that field is referenced in the Self Joins. Where would I define the extracted month in the query below?
SELECT MONTH(frcst.InvDate) AS mnth
, frcst.LineCode
, frcst.ClassCode
, cc_type
, rank
, keycust1
, keycust2
, keycust3
, sales1
, sales2
, sales3
, SUM(ship2017.GrossSales) AS gross_sales2017
, SUM(ship2017.QtyShip + ( (ship2017.QtyOrd - ship2017.QtyShip) * 0.25) ) AS frcst_qty2017
, SUM(ship2018.GrossSales) AS gross_sales2018
, SUM(ship2018.QtyShip + ( (ship2018.QtyOrd - ship2018.QtyShip) * 0.25) ) AS frcst_qty2018
, SUM(ship2019.GrossSales) AS gross_sales2019
, SUM(ship2019.QtyShip + ( (ship2019.QtyOrd - ship2019.QtyShip) * 0.25) ) AS frcst_qty2019
, SUM(ship2020.GrossSales) AS gross_sales2020
, SUM(ship2020.QtyShip + ( (ship2020.QtyOrd - ship2017.QtyShip) * 0.25) ) AS frcst_qty2020
FROM FrcstFactTbl frcst
JOIN account_hierarchy_lu account
ON frcst.AccountNumber = account.account_number
JOIN cc_type_lu cct
ON frcst.ClassCode = cct.class_code
JOIN pop_code_lu pop
ON frcst.PartNumber = pop.PartNumber
JOIN FrcstFactTbl ship2017
ON frcst.mnth = ship2017.mnth
AND frcst.LineCode = ship2017.LineCode
AND frcst.ClassCode = ship2017.ClassCode
AND frcst.AccountNumber = ship2017.AccountNumber
JOIN FrcstFactTbl ship2018
ON frcst.mnth = ship2018.mnth
AND frcst.LineCode = ship2018.LineCode
AND frcst.ClassCode = ship2018.ClassCode
AND frcst.AccountNumber = ship2018.AccountNumber
JOIN FrcstFactTbl ship2019
ON frcst.mnth = ship2019.mnth
AND frcst.LineCode = ship2019.LineCode
AND frcst.ClassCode = ship2019.ClassCode
AND frcst.AccountNumber = ship2019.AccountNumber
JOIN FrcstFactTbl ship2020
ON frcst.mnth = ship2020.mnth
AND frcst.LineCode = ship2020.LineCode
AND frcst.ClassCode = ship2020.ClassCode
AND frcst.AccountNumber = ship2019.AccountNumber
WHERE YEAR(ship2017.InvDate) = '2017'
AND YEAR(ship2018.InvDate) = '2018'
AND YEAR(ship2019.InvDate) = '2019'
AND YEAR(ship2020.InvDate) = '2020'
GROUP BY mnth, frcst.LineCode, frcst.ClassCode, cc_type, rank, keycust1, keycust2, keycust3, sales1, sales2, sales3
ORDER BY mnth

Answer 1

The only place you can use a column alias (which is what mnth is) is in the order by clause. Everywhere else you have to use the computation MONTH(frcst.InvDate) or else compute it in a sub-query.
I would use a sub-query i.e. replace this line:
FROM FrcstFactTbl frcst
With this:
FROM (
select *, MONTH(frcst.InvDate) AS mnth
from FrcstFactTbl
) frcst
And of course replace the first line:
SELECT MONTH(frcst.InvDate) AS mnth
With
SELECT mnth

Answer 2

Thank you all. Below is the query that finally worked.
SELECT
masterlist.Month AS [Month]
,masterlist.LineCode AS [Line Code]
,masterlist.ClassCode AS [Class Code]
,ct.cc_type AS [Class Code Type]
,pop.pop_code AS [Pop Code]
,ah.keycust1 AS [Key Cust1]
,ah.keycust2 AS [KeyCust2 - Territory]
,ah.keycust3 AS [Key cust3]
,ah.sales1 AS [Sales1]
,ah.sales2 AS [Sales2]
,ah.sales3 AS [Sales3]
,SUM(ff2017.[Gross]) AS [2017 Gross]
,SUM (ff2017.QtyShip + ( (ff2017.QtyOrd - ff2017.QtyShip) * 0.25) ) AS [2017 Forecast Qty]
,SUM(ff2018.[Gross]) AS [2018 Gross]
,SUM (ff2018.QtyShip + ( (ff2018.QtyOrd - ff2018.QtyShip) * 0.25) ) AS [2018 Forecast Qty]
,SUM(ff2019.[Gross]) AS [2019 Gross]
,SUM (ff2019.QtyShip + ( (ff2019.QtyOrd - ff2019.QtyShip) * 0.25) ) AS [2019 Forecast Qty]
,SUM(ff2020.[Gross]) AS [2020 Gross]
,SUM (ff2020.QtyShip + ( (ff2020.QtyOrd - ff2020.QtyShip) * 0.25) ) AS [2020 Forecast Qty]
FROM (
SELECT
DISTINCT ff.AccountNumber, MONTH(InvDate) AS Month,LineCode,ClassCode,ff.PartNumber
FROM FrcstFactTbl ff
) AS masterlist
LEFT OUTER JOIN
(
SELECT
AccountNumber
,PartNumber
,Month(InvDate) AS Month
,SUM(GrossSales) AS [Gross]
,SUM (QtyShip) AS [QtyShip]
,SUM(QtyOrd) AS [QtyOrd]
FROM FrcstFactTbl WHERE Year(InvDate)=2017
GROUP BY AccountNumber, PartNumber, Month(InvDate)
) AS ff2017 ON masterlist.Month = ff2017.Month AND masterlist.AccountNumber = ff2017.AccountNumber AND masterlist.PartNumber = ff2017.PartNumber
LEFT OUTER JOIN
(
SELECT
AccountNumber
,PartNumber
,Month(InvDate) AS Month
,SUM(GrossSales) AS [Gross]
,SUM (QtyShip) AS [QtyShip]
,SUM(QtyOrd) AS [QtyOrd]
FROM FrcstFactTbl WHERE Year(InvDate)=2018
GROUP BY AccountNumber, PartNumber, Month(InvDate)
) AS ff2018 ON masterlist.Month = ff2018.Month AND masterlist.AccountNumber = ff2018.AccountNumber AND masterlist.PartNumber = ff2018.PartNumber
LEFT OUTER JOIN
(
SELECT
AccountNumber
,PartNumber
,Month(InvDate) AS Month
,SUM(GrossSales) AS [Gross]
,SUM (QtyShip) AS [QtyShip]
,SUM(QtyOrd) AS [QtyOrd]
FROM FrcstFactTbl WHERE Year(InvDate)=2019
GROUP BY AccountNumber, PartNumber, Month(InvDate)
) AS ff2019 ON masterlist.Month = ff2019.Month AND masterlist.AccountNumber = ff2019.AccountNumber AND masterlist.PartNumber = ff2019.PartNumber
LEFT OUTER JOIN
(
SELECT
AccountNumber
,PartNumber
,Month(InvDate) AS Month
,SUM(GrossSales) AS [Gross]
,SUM (QtyShip) AS [QtyShip]
,SUM(QtyOrd) AS [QtyOrd]
FROM FrcstFactTbl WHERE Year(InvDate)=2020
GROUP BY AccountNumber, PartNumber, Month(InvDate)
) AS ff2020 ON masterlist.Month = ff2020.Month AND masterlist.AccountNumber = ff2020.AccountNumber AND masterlist.PartNumber = ff2020.PartNumber
LEFT OUTER JOIN cc_type_lu ct ON masterlist.ClassCode = ct.class_code
LEFT OUTER JOIN pop_code_lu pop ON masterlist.PartNumber = pop.PartNumber
LEFT OUTER JOIN account_hierarchy_lu ah ON masterlist.AccountNumber = ah.account_number
--ORDER BY masterlist.Month
GROUP BY masterlist.Month
,masterlist.LineCode
,masterlist.ClassCode
,ct.cc_type
,pop.pop_code
,ah.keycust1
,ah.keycust2
,ah.keycust3
,ah.sales1
,ah.sales2
,ah.sales3