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