Hierarchical (also called tree) structuring specifies that an entity can have no more than one owning entity; that is, we establish a 1:1 or 1:M relationship. The owning entity is called the parent; the owned entity, the child. A parent with no owners is called the root. There is only one root in a hierarchical model.
The customer record is the parent or the root; it has six children (2-7). As parents, 3, 4, and 5 each has three children (8-10, 11-13, and 14-16, respectively), giving the profile of a tree structure.
A parent can have many children (1:M), whereas a child can have only e parent. Elements at the ends of the branches with no children (2,5, and --16) are called leaves. Trees are normally drawn upside down, with the root at the top and the leaves at the bottom.
The hierarchical model is easy to design and understand. Some applications, however, do not conform to such a scheme. The problem is sometimes resolved by using a network-structure.
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