Understanding and distinguishing between hierarchies
A hierarchy is a system in which people, objects, or concepts are organized into a tree-like structure, with each level representing a different category or grouping of data. In modeling, hierarchies can be thought of as a series of descending one-to-many relationships.
At the top of the hierarchy sits the root node, which contains child nodes. Each child node represents a subcategory of the data contained in the parent node and may, in turn, have its own child nodes. This arrangement of nodes and their relationships is often called a tree structure or diagram. If we were to proverbially chop down such a tree and observe it laterally, we would see the hierarchy in its relational form.
Figure 17.1: A hierarchy seen in a tree (left) and relational format (right)
Hierarchies fall into three general categories depending on the variability in their levels. Let’s look at each of these categories.
