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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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