Semi additive explanation

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Semi additive explanation

kudz

Mr or Mrs. 500

Points: 542
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October 22, 2014 at 9:36 am

#397546

I've read in loads of different books/places that when we are dealing for example with an inventory the stock number is a semi additive and you can add it up by product. I really don't understand this affirmation and i would like to ask your help to understand it.
How can a stock number be additive by product ? Imagine that i have this situation
Product / Store / Date / Stock
A,LX,01-01-2014,10
A,LX,02-01-2014,5
If i sum it up by product i will have 15 for product A ? Does it make any sense ? I never had stock of 15 for the product A. When they say that is additive by product its not the same as (select sum(stock) group by product ? )
Thanks, T

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gmontanaro SSC-Addicted Points: 478 More actions · Answer 1

Semi-Additive means that it makes sense to sum it by some dimensions, but not all.

In your case, summing Products over multiple dates makes no sense, but summing by Store for a single date does.

ProductStoreDateStock

ALX01-01-201410

ASF01-01-20145

ANY01-01-20147

ALX02-01-201412

ASF02-01-20146

ANY02-01-20147

SELECTProduct

,SUM(Stock) StockLevel

FROMFactSales

GROUP BYProduct

WHEREDate='1/1/2014'

StockLevel = 22

kudz Mr or Mrs. 500 Points: 542 More actions · Answer 2

Thank you for your help 🙂

I was basically thinking that i had to ignore completely the Date dimension and i thought you would have to analyze the full table and not for a single date.

Thank you again.

gmontanaro (10/22/2014)
Semi-Additive means that it makes sense to sum it by some dimensions, but not all.
In your case, summing Products over multiple dates makes no sense, but summing by Store for a single date does.
ProductStoreDateStock
ALX01-01-201410
ASF01-01-20145
ANY01-01-20147
ALX02-01-201412
ASF02-01-20146
ANY02-01-20147
SELECTProduct
,SUM(Stock) StockLevel
FROMFactSales
GROUP BYProduct
WHEREDate='1/1/2014'
StockLevel = 22

aaron.reese SSChampion Points: 13498 More actions · Answer 3

well put G.

Just to add to the discussion. If you wanted to be able to SUM() across the date dimension you would need to record stock MOVEMENT not stock POSITION

but this would mean that if you want to know the POSITION you would have to SUM() the MOVEMENTs for all time (or at least from the last known POSITION)