ne conceptual database and is designed to meet the needs of particu External Level (View) group of users. Such a group can only have access to the data specified by its particular view. This, of course, ensures both privacy and security. The mapping between the three levels of abstraction is the task of the DBMS. When changes to the internal level(such as a change in file organi- tion)do not affect the conceptual and external levels, the system is said to provide for physical data independence. Logical data independence prevents Conceptual Level changes to the conceptual level to affect users' views. Both types of data independence are desired features in a database system Data Models a data model refers to an integrated set of tools used to describe the data Internal Level and its structure data relation and data constraints. Some data models provide a set of operators that is used to update and query the database. Data FIGURE 94.1 Data abstraction. nodels can be classified in two main categories: record-based and object based. Both classes are used to describe the database at the conceptual and external levels. with object-based data models, constraints on the data can be specified more explicitly. There are three main record-based data models: the relational, network, and hierarchical models. In the relational model, data at the conceptual level is represented as a collection of interrelated tables. These tables are normalized so as to minimize data redundancy and update anomalies. In this model, data relationships ar implicit and are derived by matching columns in tables In the hierarchical and network models, the data represented as a collection of records and data relationships are explicit and are represented by links. The lifference between the last two models is that in the hierarchical model, data is represented as a tree structure, while it is represented as a generalized graph in the network model In hierarchical and network models, the existence of physical pointers(links)to link related records alloy an application program to retrieve a single record at a time by following the pointer's chain. The process of following the pointer's chain and selecting one record at a time is referred to as navigation. In nonnavigational models such as the relational model, records are not related through pointer's chains, but relationships are established by matching columns in different tables The hierarchical and network models require the application programmer to be aware of the internal structure of the database. The relational model, on the other hand, allows for a high degree of physical and logical data independence. Earlier DBMSs were for the most part navigational systems. Because of its simplicity and strong theoretical foundations, the relational model has since received wide acceptance. Today, most DBMSs are based on the relational model Other data models include a popular high level conceptual data model, known as the Entity-Relationship ER)model. The ER model is mainly used for the conceptual design of databases and their applications. The ER model describes data as entities, attributes, and relationships. An entity is an"object"in the real world with an independent existence. Each entity has a set of properties, called attributes, that describes it. A relationship is an association between entities. For example, a professor entity may be described by its name, age, and salary and can be associated with a department entity by the relationship"works for With the advent of advanced database applications, the ER modeling concepts became insufficient. This has led to the enhancement of the ER model with additional concepts, such as generalization, categories, and inheritance, leading to the Enhanced-ER or EER model Relational databases The relational model was introduced by E. F Codd [1970]. Since the theoretical underpinnings of the relational model have been well defined it has become the focus of most commercial dbmss In the relational model, the data is represented as a collection of relations. To a large extent, each relation can be thought of as a table. The example of Fig. 94. 2 shows part of a university database composed of two e 2000 by CRC Press LLC© 2000 by CRC Press LLC the conceptual database and is designed to meet the needs of a particular group of users. Such a group can only have access to the data specified by its particular view. This, of course, ensures both privacy and security. The mapping between the three levels of abstraction is the task of the DBMS. When changes to the internal level (such as a change in file organi￾zation) do not affect the conceptual and external levels, the system is said to provide for physical data independence. Logical data independence prevents changes to the conceptual level to affect users’ views. Both types of data independence are desired features in a database system. Data Models A data model refers to an integrated set of tools used to describe the data and its structure, data relationships, and data constraints. Some data models provide a set of operators that is used to update and query the database. Data models can be classified in two main categories: record-based and object￾based. Both classes are used to describe the database at the conceptual and external levels. With object-based data models, constraints on the data can be specified more explicitly. There are three main record-based data models: the relational, network, and hierarchical models. In the relational model, data at the conceptual level is represented as a collection of interrelated tables. These tables are normalized so as to minimize data redundancy and update anomalies. In this model, data relationships are implicit and are derived by matching columns in tables. In the hierarchical and network models, the data is represented as a collection of records and data relationships are explicit and are represented by links. The difference between the last two models is that in the hierarchical model, data is represented as a tree structure, while it is represented as a generalized graph in the network model. In hierarchical and network models, the existence of physical pointers (links) to link related records allows an application program to retrieve a single record at a time by following the pointer’s chain. The process of following the pointer’s chain and selecting one record at a time is referred to as navigation. In nonnavigational models such as the relational model, records are not related through pointer’s chains, but relationships are established by matching columns in different tables. The hierarchical and network models require the application programmer to be aware of the internal structure of the database. The relational model, on the other hand, allows for a high degree of physical and logical data independence. Earlier DBMSs were for the most part navigational systems. Because of its simplicity and strong theoretical foundations, the relational model has since received wide acceptance. Today, most DBMSs are based on the relational model. Other data models include a popular high level conceptual data model, known as the Entity-Relationship (ER) model. The ER model is mainly used for the conceptual design of databases and their applications. The ER model describes data as entities, attributes, and relationships. An entity is an “object” in the real world with an independent existence. Each entity has a set of properties, called attributes, that describes it. A relationship is an association between entities. For example, a professor entity may be described by its name, age, and salary and can be associated with a department entity by the relationship “works for”. With the advent of advanced database applications, the ER modeling concepts became insufficient. This has led to the enhancement of the ER model with additional concepts, such as generalization, categories, and inheritance, leading to the Enhanced-ER or EER model. Relational Databases The relational model was introduced by E. F. Codd [1970]. Since the theoretical underpinnings of the relational model have been well defined, it has become the focus of most commercial DBMSs. In the relational model, the data is represented as a collection of relations. To a large extent, each relation can be thought of as a table. The example of Fig. 94.2 shows part of a university database composed of two FIGURE 94.1 Data abstraction