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Software Engineering
Notes While a node can have any number of child nodes, it can have only one parent node. A node in
the tree represents a record. All attributes of a specific record are listed under an entity type.
Entity types are related to each other using 1: N mapping. A typical hierarchical data model
looks like the one shown in figure 14.2.
Figure 14.2: A Hierarchical Data Model
Prior to the development of the first Database Management System (DBMS), access to data was
provided by application programs that accessed flat files. The data integrity problem and the
inability of such file processing systems to represent logical data relationships lead to the first
data model: the hierarchical data model. This model, which was implemented primarily by
IBM’s Information Management System (IMS) only allows one-to-one or one-to-many
relationships between entities. Any entity at the many end of the relationship can be related
only to one entity at the one end.
Example: A hierarchical data model would be if an organization had records of employees
in a table (entity type) called “Employees”. In the table there would be attributes/columns such
as First Name, Last Name, Job Name and Wage. The company also has data about the employee’s
children in a separate table called “Children” with attributes such as First Name, Last Name, and
date of birth. The Employee table represents a parent segment and the Children table represents
a Child segment. These two segments form a hierarchy where an employee may have many
children, but each child may only have one parent.
Self Assessment
Fill in the blanks:
1. A relation can be thought as a table of values, each row in the table represents a collection
of related…………………….
2. A ………………………. is not just a way of structuring data: it also defines a set of operations
that can be performed on the data.
3. A data model captures the essence of ……………………….. with the data items.
4. In relational model terminology, a row in the table is called a ………………….. and a
column header is called an attribute.
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