Lecture 01 - Chapter 01.pdf

Chapter 01 Introduction  Introduction A database-management system (DBMS) is a collection of interrelated data and a set of programs to access those data. The collection of data, usually referred to as the database, contains information relevant to an enterprise. T...

Chapter 01 Introduction  Introduction A database-management system (DBMS) is a collection of interrelated data and a set of programs to access those data. The collection of data, usually referred to as the database, contains information relevant to an enterprise. The primary goal of a DBMS is to provide a way to store and retrieve database information that is both convenient and efficient.  Introduction [Cont.] Database systems are designed to manage large bodies of information. Management of data involves: – Defining structures for storage of information. – Providing mechanisms for the manipulation of information. – In addition, the database system must ensure the safety of the information stored, despite system crashes or attempts at unauthorized access. 1.1 DATABASE-SYSTEM APPLICATIONS  Database-System Applications Enterprise Information. Banking and Finance. Universities. Airlines. Telecommunication: 1.2 PURPOSE OF DATABASE SYSTEMS  file-processing system Before database management systems (DBMSs) were introduced, organizations usually stored information it in operating system files.  file-processing system disadvantages 1. Data redundancy and inconsistency. 2. Difficulty in accessing data. 3. Data isolation. 4. Integrity problems. 5. Atomicity problems. 6. Concurrent-access anomalies. 7. Security problems.  Data redundancy and inconsistency Since different programmers create the files and application programs over a long period, the various files are likely to have different structures and the programs may be written in several programming languages. Moreover, the same information may be duplicated in several places (files).  Difficulty in accessing data. Conventional file-processing environments do not allow needed data to be retrieved in a convenient and efficient manner.  Data isolation Because data are scattered in various files, and files may be in different formats, writing new application programs to retrieve the appropriate data is difficult.  Integrity problems The data values stored in the database must satisfy certain types of consistency constraints.  Atomicity problems A computer system, like any other device, is subject to failure. In many applications, it is crucial that, if a failure occurs, the data be restored to the consistent state that existed prior to the failure. Concurrent-access anomalies  Security problems Not every user of the database system should be able to access all the data. 1.3 VIEW OF DATA  View of the data A major purpose of a database system is to provide users with an abstract view of the data. – That is, the system hides certain details of how the data are stored and maintained.  Data Abstraction For the system to be usable, it must retrieve data efficiently. – The need for efficiency has led designers to use complex data structures to represent data in the database. Since many database-system users are not computer trained, developers hide the complexity from users through several levels of abstraction, to simplify users’ interactions with the system: – Physical level. – Logical level. – View level.  Data Abstraction / Physical level The lowest level of abstraction describes how the data are actually stored. The physical level describes complex low- level data structures in detail.  Data Abstraction / Logical Level What data are stored in the database – and what relationships exist among those data.  Data Abstraction / View Level Describes only part of the entire database. The system may provide many views for the same database.  Instances and Schemas The collection of information stored in the database at a particular moment is called an instance of the database. The overall design of the database is called the database schema.  Physical data independence Application programs are said to exhibit physical data independence if they do not depend on the physical schema, and thus need not be rewritten if the physical schema changes. End of Lecture 01