INF2603 answers-to-review-questions.

Answers to Review Questions 1. Name and discuss the different levels of data abstraction as defined by ANSI/SPARC. Conceptual Located at the abstraction apex, the conceptual model represents a global view of the data. It forms the basis of the conceptual schema, which provides a relatively easily understood bird's-eye view of the data environment. Since the conceptual model focuses on the main data objects and avoids detail, it exhibits both software and hardware independence. The most widely used conceptual model is the Entity Relationship (E-R) model, which yields the basic database blueprint. Internal The internal model represents the adaptation of the conceptual model to a specific DBMS. Basically, the internal model requires the database designer to match the conceptual model's characteristics and constraints to those of the selected hierarchical, network, or relational DBMS. Therefore, although it is still hardware-independent, it is software-dependent. External The external model represents the applications programmer's view of the data environment. Its use requires that the modeler subdivide a universal set of requirements and constraints into functional modules, each represented by its own external model. (The modules correspond to business units such as production, sales, personnel, and so on.) Each external model is represented by its own external schema. Each business unit is thus represented by an external model that includes that unit's entities, the relationships between the entities, and its constraints. Since external models are defined for a specific DBMS, they are DBMS (software)-dependent, but hardware-independent. The use of external schemas has several important advantages:  Using database subsets makes it easier to view the specific application program requirements.  The subsets make it easier to identify specific data required to support a given business unit's operations.  They make it easier to examine feedback about the conceptual model's adequacy.  It is more difficult to damage the entire database when each module employs only the required data subset. Downloaded by Shaun Nyamanjerere () lOMoARcPSD| Physical The physical model operates at the lowest level of abstraction. It describes the way data will be saved on storage media such as disks or tapes. Because the physical model requires the specification of physical storage devices and the access methods required to reach the data located on those storage devices, the physical model is both hardware-and software-dependent. (See section 3.2.4.) 2. What are the main building modules of the Entity Relationship model? Discuss each one. The E-R model contains the following components: Entities are used to represent real world objects such as professor, class, student, etc. Actually, the E-R model uses entity sets, which are the grouping of related entities. For example, the entity set named STUDENT is composed of specific entities such as Anne R. Morowski, William F. Achero, and so on. So, when E-R models are used, the word entity actually refers to entity set. Entity types include weak and composite entities. Weak entities have two characteristics: 1. They are existence-dependent, i.e., they cannot exist without some other entity. (Example: DEPENDENT cannot exist without EMPLOYEE.) 2. The weak entity's primary key is partially or wholly derived from the parent side of the relationship. Composite entities are used to transform M:N relationships into sets of 1:M relationships. The composite entity's primary key consists of the combination of primary keys from the entities it connects. Finally, entities may be classified as supertypes and subtypes. In effect, we can create a generalization hierarchy to represent entities that share common characteristics. For example, all EMPLOYEE entities share attributes such as NAME, ADDRESS, HOME_PHONE, etc. However, special employees such as pilots have attributes, such as FLT_TIME, not shared by other employees. Thus PILOT becomes a subtype of the supertype EMPLOYEE. (See the detailed discussion in section 3.3.12!) Attributes are the characteristics that define an entity. For example, the attributes NAME, ADDRESS, PHONE, and MAJOR are some of the characteristics of the STUDENT. Some attributes perform the crucial role of identifying each entity uniquely. Such attributes form the entity's primary key.