Factless Fact Table: A fact table with a key section but without a measure
section.  There are 2 types of factless fact table:

1. Event Tracking Tables: A table that records an event. Ex: A table
that records student attendance.  It contains date_key, student_key,
teacher_key, course_key and without measure section.

2. Coverage Tables: A table to answer questions about things that
didn't happen.  Ex: An ordinary fact table canot answer questions like
"Which products were on promotion that did not sell?", because it contains only
the records of products that did sell. A table contains date_key, product_key,
store_key, promo_key and without measure section is used to answer such
questions.