Comparing apples to oranges

Setting: requirements gathering meeting.

User: (firm) My data cannot be aggregated.

Analyst: Cannot or should not?

User: (puzzled) Cannot.

Analyst: Why not?

User: Because you cannot compare apples to oranges.

Analyst: No? Why can’t you compare their weight, volume, taste etc.?

User: Hmm. (thinking) OK, but you cannot add them.

Analyst: Well, in a way you can: 5 apples and 3 oranges are 8 pieces of fruit.

User: Hmm. (thinking) Maybe you’re right. (enthusiastic) That would give me some of those key figures I have to report every month. Never thought I would be able to get that information from the data warehouse. Now, can we define the rules when (not) to aggregate data and what aggregations to apply?

Analyst: Yes we can!