What is the difference between fact-less fact and fact table?

Question

What is the exact difference between fact-less fact and fact table ? I have perused couple of articles but they weren't cogent

Complete

Answer 1

A factless fact table is a fact table that does not have any measures, i.e. any numeric fields that can be aggregated.

For example, if you are modelling product sales, you can have a Sales fact table that will contain the dimension keys and, for example, the "amount" value/measure, to record the amount sold. This allows you to run aggregations on the "amount" field to answer questions like "what's the average sales amount?" or "what was the maximum sales amount?".

However, if the same Sales table does not include the "amount" field, you can only use the table to answer questions like "how many sales did we do?". If the table has keys for the Product and Date dimensions you could answers questions like "how many sales of product X did we have in date Y?".

Factless fact tables are useful when you only care about recording that some event occurred, e.g. for counting occurrences of an event when you don't have or don't need any metrics/measures regarding that event.

I'm not 100% sure, but I think this concept originated from the Kimball methodology.

Escapement

That was a brilliant explanation. Only one thing I would like to know that in a sales factless fact table if amount field is missing but still we can aggregate to know how many sales we made. Then why do we say that factless fact table doesn't allows us to run aggregations ? Can you elucidate this – Complete

I understand your point; "factless" is just the nomenclature defined to represent transactions that are important to record but that have no other (relevant) quantitative data. In that case, we're only counting rows in a table, and not running aggregations over one of its fields. Perhaps "measureless fact table" would have been a better name :) I believe this concept and naming originated in the Kimball methodology: kimballgroup.com/2011/04/…. – Escapement