A factless fact table is fact table that does not contain fact.They contain only dimesional keys and it captures events that happen only at information level but not included in the calculations level.just an information about an event that happen over a period.
A factless fact table captures the many-to-many relationships between dimensions, but contains no numeric or textual facts. They are often used to record events or coverage information. Common examples of factless fact tables include:
Tracking student attendance or registration events
Tracking insurance-related accident events
Identifying building, facility, and equipment schedules for a hospital or university
Factless fact tables are used for tracking a process or collecting stats. They are called so because, the fact table does not have aggregatable numeric values or information.There are two types of factless fact tables: those that describe events, and those that describe conditions. Both may play important roles in your dimensional models.