Database Models

INTRODUCTION TO DATABASE MODELS

The quest for better data management has led to several different ways of solving the file system’s critical shortcomings. The resulting theoretical database constructs are represented by various database models.

A database model is a collection of logical constructs used to represent the data structure and the data relationships found within the database.

Database models can be grouped into two categories:

1. Conceptual Model
2. Implementation Model

Conceptual Model

The conceptual models use three types of relationships to describe associations among data:

1. One-to-many relationship: A painter paints many different paintings, but each one of them is painted by only that painter.
2. Many-to-many relationship: An employee might learn many job skills, and each job skill might be many employees.
3. One-to-one relationship: A retail company’s management structure may require that each one of its stores is managed by a single employee. In turn, each store manager who is an employee only manages a single store. Therefore, the relationship is one to one.

Implementation Model

An implementation model places the emphasis on how the data are represented in the database or on how the data structures are implanted to represent what is modeled. Implementation models include:

i.) Hierarchical Database Model
ii.) Network Database Model
iii.) Relational Database Model and
iv.) Entity Relationship Database Model

i.) Hierarchical Database Model

North American Rockwell was the prime contractor for the Apollo project, which culminated in a moon landing in 1969. Bringing such a complex project to a successful conclusion, this process requires the management of millions of parts. Information concerning the part was generated by a complex computer file system.

^{Fig: Hierarchical Database Model}

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When North America Rockwell began to develop its own database system, an audit of computer tapes revealed that over 60% of the data were redundant (repeated). The problems caused by data redundancy forced North American Rockwell to develop an alternate strategy for managing such huge data quantities.

Borrowing parts of existing database concepts, they developed software known as GUAM (Generalized Update Access Method), which was based on the recognition that the many smaller parts would come together as components of still larger components and so on until all the components came together in the final unit.

In the mid-sixties, IBM joined North American Rockwell to expand the capabilities of GUAM replacing the computer tape medium with more up-to-date disk computer storage which allows the introduction of complex pointer system. The results of the joint Rockwell-IBM effort become known as the Information Management System (IMS).

Advantages:

1. The relationship between various layers is logically simple.
2. This system provides a tough database security.
3. Hierarchical database system maintains data independence i.e. if a data is altered in one table, it does not affect the other location.
4. There is always a parent-child relationship and data integrity is maintained.
5. For a large volume of data and 1:M relationship, it is a very efficient model.

Disadvantages:

1. The physical implementation of the database is complicated.
2. Alternation in database structure is difficult to manage.
3. Structural independence exists when changes in the database structure do not affect the DBMS’.
4. Application programming use complexity which may exist in many cases.
5. Implementation limitations due to the incapability of managing the relation such as M:N.
6. It lacks in DDL and DML standard of commands.

ii.) Network Database Model

Network database model was created to represent complex data relationships more effectively than the hierarchical model, which could improve database performance and impose a database standard. The lack of database standards was troublesome to programmers and application designers because it made database designs and applicationsare less portable.

In many aspects, the network database model resembles the hierarchical database model. It also uses the same principle of 1:M relationship. However, quite unlike the hierarchical model, the network model allows a record to have more than one parent. Therefore, the commonly encountered relationships can be handled easily by the network database model.

^{Fig: Network Database Model}

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Using network database terminology, a relationship is called a set. Each set is compared to at least two record types: an owner record that is equivalent to the hierarchical model’s parent and a member record that is equivalent to the hierarchical model’s child. A set represents a 1:M relationship between the owner and the member.

Advantages:

1. The database is conceptually simple in structure.
2. It can handle more relationship types.
3. It has data access flexibility such as accessing its own record and all the member records in the set.
4. Promotes database integrity since owner record and member records are defined precisely.
5. Changes in the data integrity since owner record and member records are defined precisely.
6. Changes in the data do not require a change in access methods which is the benefit of data independence.
7. It confirms the standards such as DDL (Data Definition Language) and DML.

Disadvantages:

1. Since, the system is complex the system navigators, database administrators, programmer and end users must be very familiar with the internal structure in order to access the database. Lack of structural independence
2. Although, the network mode achieves data independence, it still does not produce structural independence.

iii.) Relational Database Model

The Relational Model, first developed by E.F. Codd (of IBM) in 1970, represented a major breakthrough for both users and designers. To use an analogy, the relational model produced an “automatic transmission” database to replace the “standard transmission” database that preceded it.

Its conceptual simplicity set the stage for a genuine database revolution. Codd’s work was considered ingenious but impractical in 1970. The relational model’s conceptual simplicity was brought at the expense of computer overhead; computers lacked the power to implement the relational model.

^{Fig: Relational Database Model}

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The relational database model is implemented through a very sophisticated relational database management system (RDBMS). The RDBMS performs the same basic functions provided by the hierarchical and network DBMS system plus a host of other functions that make the relational database model easier to understand and to implement.

Advantages:

1. Data access paths are irrelevant to relational database designers, programmers and end users. It is possible to change database structure without changing the method of accessing data.
2. This model is simpler and possesses improved conceptual simplicity.
3. It provides easier database design, implementation, management and use.
4. It possesses ad-hoc query capabilities.
5. It possesses a powerful database management system.

Disadvantages:

1. Substantial hardware and system software overhead due to the complex structure of database hidden procedures within the system.
2. In microcomputer based systems, it has a poor design and implementation is made easy.
3. Many promote “islands of information” problems since some database are isolated from other locations or system complexity.

iv.) Entity Relationship Database Model

The relational database model’s conceptual simplicity made it possible to expand the database’s scope. Therefore, the introduction of relational database technology triggered a demand for more and increasingly complex transactions and information.

In turn, the rapidly increasing transaction and information requirement created the need for more complex database implementation structures, thus, creating the need for more effective database design tools.

^{Fig: Entity Relationship Database Model}

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Complex design activities requires conceptual simplicity to yield successful results. Although the relational database model was a vast conceptual improvement over the hierarchical and network database models, it still lacked the features that which might make it an effective database design tool because it is easier to examine structures graphically than to describe them in text form. Database designers find it desirable to use a graphical tool in which entities and their relationships can be pictured.

Advantages

1. It yields a particularly easily viewed and understood conceptual view of a database’s main entities and their relationships.
2. The ERM gives the designers visual representation very clearly.
3. The ERM has become one of significant effective communication tools in RDBMS.
4. The ERM is well integrated with the relational database model.

Disadvantages

1. Limited constraint representation due to limited model.
2. The relationship between attributes within entities cannot be represented. Example, there is no way of completed hours and classification of a student.
3. It lacks in data manipulation language or commands.
4. The models becomes crowded due to more presence of entities.

References:

Khanal, R.C. Khanal, R.C. Computer Concept for XII. Pashupatigriha Marga, Thapathali, Kathmandu, Nepal: Ekta Books Distributors Pvt. Ltd., 2010. 51-56.

Adhikari,Deepak Kumar.,et.al., Computer Science XII,Asia Publication Pvt.Ltd