Objectives: Introduction Over View of System Analysis and Design

A one-to-one (1:1) relationship is an association between two entities. For example, in our culture, a husband is allowed on wife (at a time) and vice versa, and an employee has one social security number. A one–to-many (M) relationships describes an entity that may have two or more entities related to it. For example, a father may have many children, and an employee may have many skills. A many-to-many (MM) relationship describes entities that may have many relationship in both directions. For example, children may have many toys, and students may have many courses.

7.7 Types of Data Structure
Data structuring determines whether the system can create 1:1, M, or MM relationships among entities. Although all DBMSs have a common approach to data management, they differ in the way they structure data. There are three types of data structure hierarchical, network and relational.

7.7.1 Hierarchical Structuring
Hierarchical (also called tree) structuring specifies that an entity can have no more than one owning entity, that is we can establish a 1:1 or 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. For example, a parent can have many children (M, whereas a child can have only one parent. Elements at the ends of the branches with no children 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, such as fora firm dealing in sale of spare parts being manufactured by more than one company. Thus, we would have anon- hierarchical structure, which complicates programming or the DBMS description. The problem is resolved by using a network structure.

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