Ragged hierarchies

Ragged hierarchies are another common design problem to deal with when building an Analysis Services dimension. The hierarchies we've dealt with so far can be easily separated out into distinct levels and can be thought of as pyramid-shaped: all of members on the hierarchy have at least one child, except the members on the lowest level, which have no children at all. Ragged hierarchies, on the other hand, are bush-shaped. The members at any given level may or may not have children. Common examples of ragged hierarchies are those that represent a chart of accounts or the organizational structure of a company.

Parent/child hierarchies

One way of modeling a ragged hierarchy in a data warehouse is with a table with a self-join: every row in the table represents an item somewhere on the hierarchy, and every row has a key column and a foreign key that joins back onto the key column in order to store the key of the parent item. Here's an example taken from the Adventure Works...