Adding a new computed variable back to main dataset in SQL - sql

Question

I am trying to compute a variable (say last_week) and add it back to my main dataset (say new_j). I managed to join it to new_j. However, if I want to use that variable (last_week) now for further calculations, it does not recognise it. Here's my code:
SELECT [Weekkey] AS weekkey
,[article / colour] as prod_id
,[Current MP Department No/Desc] as prod_dept
,[Total Stock] as total_stock
INTO #new_j
FROM [J_20160831] --(that’s the db in server and I created a temp db #new_j)
SELECT prod_id, max(weekkey) as last_week
into #lastweeksales
FROM #new_j
group by prod_id
select *
from #new_j
left join #lastweeksales
on #lastweeksales.prod_id = #new_j.prod_id
So, I joined both successfully and if I run this code, I see column last_week. Now what I want to do is this:
select *
,case
when last_week = max(weekkey) then total_stock
else 0
end as last_stock_position
from #new_j
But it says last_week is not found in new_j. I also tried #lastweeksales.last_week instead of just last_week in the last bit of code, but it didn't either. What's the best way out here? Moreover, is there a better way to do it instead?. The output I am looking to have at the end is a table with these variables: WeekKey, prod_dept, prod_id, total_stock, last_week, last_stock_position
Thanks for the help!!! Much appreciate it.

Answer 1

This normal behaviour of joins..
by selecting this
select * from #new_j left join #lastweeksales
on #lastweeksales.prod_id = #new_j.prod_id'
all the columns of newj and lastweekales will be displayed in same order (first new_j columns and then lastweeksales columns ).So 'last_week' is the last column of lastweeksales.
Secondly,
select *,
case when last_week = max(weekkey) then total_stock
else 0
end as last_stock_position
from #new_j
in above query,your are selecting 'last_week' column which belongs to the table #lastweeksales.
Be careful while selecting the columns.
I guess your expecting,
select a.WeekKey, a.prod_dept, a.prod_id, a.total_stock, b.last_week,
case
when b.last_week = max(a.weekkey) then total_stock
else 0
end as last_stock_position
from #new_j as a
left join #lastweeksales as b
on b.prod_id = a.prod_id
group by a.weekkey,a.prod_dept,a.prod_id,a.total_stock,b.last_week