• A factless fact table is a fact table that does not have any measures.
• i.e. it captures the many-to-many relationships between dimensions but contains no numeric or textual facts.
• It is essentially an intersection of dimensions.
• On the surface, a factless fact table does not make sense, since a fact table is, after all, about facts.
• However, there are situations where having this kind of relationship makes sense in data warehousing.
• For example:
o Think about a record of student attendance in classes.
o In this case, the fact table would consist of 3 dimensions: the student dimension, the time dimension, and the class dimension.
o This factless fact table would look like the following:
Factless fact tables offer the most flexibility in data warehouse design.
For example, one can easily answer the following questions with this factless fact table:
• How many students attended a particular class on a particular day?
• How many classes on average does a student attend on a given day?
Without using a factless fact table, we will need two separate fact tables to answer the above two questions.