Chapter Summary - CHAPTER. Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION. Databas

A corporation’s operational data is a corporate resource, an asset that is of value to the organization and incurs cost. A database, which is shared by many users, protects operational data by providing data security, integrity constraints, reliability controls, and professional management by a DBA.

Data means facts, while information is processed data. There are four levels of discussion about data:reality(the real world) containing themini-

world (Universe of Discourse) that we are modeling, the abstract

conceptual modelof the miniworld, thelogical modelorintentionof the database with its metadata(data about data), and thedatabase extension, withdata instances.

A staged approach to database design is a top-down approach that allows the designer to develop a conceptual model that mirrors the operations of the organization, allows changes, supports many user views, is independent of physical implementation, and does not depend on the model of a particular DBMS. This approach allows the database to evolve as needs change. In staged database design the designer must analyze the user environment, develop a conceptual data model, choose the DBMS, create the logical model by mapping the conceptual model to the data model of the DBMS, develop the internal and physical models, evaluate the physical

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2.8 Chapter Summary 57

model, perform tuning if needed, and implement the physical model. Steps may be repeated until the design is satisfactory.

Design methodologies can be general techniques or commercial products such as CASEpackages. Adata dictionarycan beintegrated(the DBMS system catalog) orfreestanding(independent of a DBMS). A data dictio- nary is a valuable tool for collecting and organizing information about data. The DBA must be technically competent, a good manager, and have excel- lent interpersonal and communication skills. He or she has primary responsibility for planning, designing, developing, and managing the operating database.

Standard database architecture uses three levels of abstraction:external,logi- cal, andinternal. Anexternal viewis the model seen by a particular user. An

external schemais an external model description written in thedata defini- tion language, which is part of thedata sublanguagefor the DBMS being used. Theuser interfacecreates the user’s working environment and hides lower levels from the user. Thelogical schemais a complete DDL description of the logical model. It specifies the information content of the database, including all record types and fields. It is derived from the conceptual model. A good conceptual model results in a logical model that is easy to implement and supports the desired external views. Thelogical record interface is a boundary below which the logical level cannot see. Theinternal schemais a DDL description of the internal model. It specifies how data is represented, how records are sequenced, what indexes and pointers exist, and what hashing scheme is used. Thestored record interfaceis the boundary below which the DBMS does not see. The operating system should be responsible for all physical details below this interface, including creating aphysical record interface

for itself to handle low-level physical details such as track placement. The

external/logical mappingtells how the external views correspond to the logical items. Thelogical/internalmapping tells how logical items correspond to internal ones.Data independencemakes each level immune to changes on lower levels.Logical data independencemeans that the logical level can be changed without changing external views.Physical data independence

means internal and physical changes will not affect the logical model.

Some data models are the entity-relationship,object-oriented,object- relational, and semistructured models. There are also record-based

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models, including the relationalas well as the older networkand hierar- chicalmodels. The entity-relationship model uses E-R diagramsto show entity sets, attributes of entities, relationship sets, and descriptive attributes of relationships. The relational model uses tables to represent data and relationships. The object-oriented model uses the concept of a class, having attributes and methods. An object is created as an instance of a class. Object-oriented databases contain persistent objects. Object- relational databases are extensions of relational model databases to allow complex objects and methods. Semistructured databases consist of nodes that are self-describing using XML.

In document CHAPTER. Jones & Bartlett Learning, LLC NOT FOR SALE OR DISTRIBUTION. Database Planning and Database Architecture (Page 32-34)