How to calculate row using different value from joined table - sql

Question

I need to run an update on a SQL table, using a value from a different (but almost identical) row in the same table, to show how different sales scenarios play out.
e.g.
Starting with:
ITEM, SITE, SUPPLIER, SCENARIO, SALES_VOLUME, PRICE, TOTAL_SALES
1 , A , X , S1 , 100 , 10 , 1000
2 , A , Y , S1 , 25 , 20 , 500
1 , A , X , S2 , {blank} , 20 , 0
I would like to update to show:
ITEM, SITE, SUPPLIER, SCENARIO, SALES_VOLUME, PRICE, TOTAL_SALES
1 , A , X , 1 , 100 , 10 , 1000
2 , A , Y , 1 , 25 , 20 , 500
1 , A , X , 2 , {blank} , 20 , 2000
So basically, if SCENARIO = S2, I need to recalculate using SALES_VOLUME where SCENARIO = S1
I tried the following, but it didn't work - I think because I'm trying to specify both =1 and !=1 in the same lookup.
UPDATE TABLE1
SET [TOTAL_SALES] = (t1.[PRICE] * t2.[SALES_VOLUME])
FROM TABLE1 t1
inner join TABLE1 t2
on t1.[ITEM] = t2.[ITEM]
and t1.[SITE] = t2.[SITE]
and t1.[SUPPLIER] = t2.[SUPPLIER]
and t1.[SCENARIO] = 'S1'
WHERE t1.[SCENARIO] != 'S1'
I don't think I'm too far off, but just feel I'm missing something.
Any pointers would be gratefully received. :)

Answer 1

You may try this -
UPDATE t1
SET [TOTAL_SALES] = (t1.[PRICE] * t2.[SALES_VOLUME])
FROM TABLE1 t1
inner join TABLE1 t2
on t1.[ITEM] = t2.[ITEM]
and t1.[SITE] = t2.[SITE]
and t1.[SUPPLIER] = t2.[SUPPLIER]
and t1.[SCENARIO] = 'S2'
WHERE t2.[SCENARIO] = 'S1';