Semi-additive facts

What are semi-additive facts?

Well, facts that can be aggregated across a few dimensions, but not all.

Take the case of an inventory fact table.

We track inventory every day for every product at every warehouse.

To get the inventory on a given day for all products, I aggregate the inventory fact measures across the product dimension filtered for that particular day. The following query will give the inventory for every product for today.

Select product_dim.name, sum(Inv_fact.inventory_qty)
from
product_dim, inv_fact, date_dim
where
date_dim.date_key= inv_fact.date_key and
date_dim.calendar_dt=sysdate and
product_dim.product_key = inv_fact.product_key
group by product_dim

However, I cannot add all the inventory quantities every day for a product, say for a month and get anything meaningful. Aggregation across time is meaningless. (of course, you can use it to calculate the average inventory per day)

Such facts that can be aggregated across certain dimensions but not across all dimensions are called Semi-additive or partial facts.