The star schema (see “Dimensional Modeling, Star, and Snowflake Schemas” on page 187), which is the most common form of denormalization used in contemporary data warehousing, cannot handle every kind of relationship that can exist comfortably in a fully-normalized environment. Recursive relationships are one such example. Recursion, as the term is generally used in computer science, is only a small subset of the recursive function theory of formal logic.
A recursive relationship exists when the parent of a member in a hierarchical relationship is also a member of the same entity. As demonstrated by the following figure, there are two ways that this can manifest itself: with only a single parent or with multiple parents:
The most commonly used example of a single-parent recursive relationship is an employee table, where both an employee and its manager have rows. From an E-R perspective, you would say a manager has employees. But managers, too, are employees. This also means that managers can have managers who are employees, and so on.
In the diagram, the single-parent recursive relationship is a customer table in which a customer can be a customer of yet another customer in the table. The classic multiple-parent recursive relationship is the bill of material. The diagram shows an example in which multiple organizations can have multiple organizational affiliations. Project work breakdown hierarchies are another common example of a a multiple parent recursive structure.
In a recursive structure, there can be an unlimited number of levels without knowing how many levels each member hierarchy currently has or potentially can have. One hierarchy have only two levels, while another might be 15 levels deep. Herein lies the limitation of the star schema for handling recursive relationships: it requires a fixed number of levels, because each level is set up by a series of fixed columns in a dimension table. Because you do not know the number of levels in a recursive structure, you cannot predefine the columns.
The most critical entities in an enterprise data model frequently have recursive structures. Organizational hierarchies such as internal, customer, supplier, and competitor entities are usually recursive relationships.