Summary of database management system.

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Summary

• A database-management system (DBMS) consists of a collection of interrelated data and a collection of programs to access that data. The data describe one particular enterprise.

• The primary goal of a DBMS is to provide an environment that is both convenient and efficient for people to use in retrieving and storing information.

• Database systems are ubiquitous today, and most people interact, either directly or indirectly, with databases many times every day.

• Database systems are designed to store large bodies of information. The management of data involves both the definition of structures for the storage of information and the provision of mechanisms for the manipulation of information. In addition, the database system must provide for the safety of the information stored, in the face of system crashes or attempts at unauthorized access. If data are to be shared among several users, the system must avoid possible anomalous results.

• 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.

• Underlying the structure of a database is the data model: a collection of conceptual tools for describing data, data relationships, data semantics, and data constraints. The entity-relationship (E-R) data model is a widely used data model, and it provides a convenient graphical representation to view data, relationships and constraints. The relational data model is widely used to store data in databases. Other data models are the object-oriented model, the object- relational model, and semistructured data models.

• The overall design of the database is called the database schema. A database schema is specified by a set of definitions that are expressed using a data- definition language (DDL).

• A data-manipulation language (DML) is a language that enables users to access or manipulate data. Nonprocedural DMLs, which require a user to specify only what data are needed, without specifying exactly how to get those data, are widely used today.

• Database users can be categorized into several classes, and each class of users usually uses a different type of interface to the database.

• A database system has several subsystems.

The transaction manager subsystem is responsible for ensuring that the database remains in a consistent (correct) state despite system failures. The transaction manager also ensures that concurrent transaction executions proceed without conflicting.

The query processor subsystem compiles and executes DDL and DML statements.

The storage manager subsystem provides the interface between the low- level data stored in the database and the application programs and queries submitted to the system.

Exercises 23

• Database applications are typically broken up into a front-end part that runs at client machines and a part that runs at the back end. In two-tier architectures, the front-end directly communicates with a database running at the back end. In three-tier architectures, the back end part is itself broken up into an appli- cation server and a database server.

Review Terms

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Exercises

List four significant differences between a file-processing system and a DBMS.

This chapter has described several major advantages of a database system. What are two disadvantages?

Explain the difference between physical and logical data independence.

List five responsibilities of a database management system. For each responsi- bility, explain the problems that would arise if the responsibility were not dis- charged.

What are five main functions of a database administrator?

List seven programming languages that are procedural and two that are non- procedural. Which group is easier to learn and use? Explain your answer.

List six major steps that you would take in setting up a database for a particular enterprise.

Consider a two-dimensional integer array of size n × m that is to be used in your favorite programming language. Using the array as an example, illustrate the difference (a) between the three levels of data abstraction, and (b) between a schema and instances.

Bibliographical Notes

We list below general purpose books, research paper collections, and Web sites on databases. Subsequent chapters provide references to material on each topic outlined in this chapter.

Textbooks covering database systems include Abiteboul et al. [1995], Date [1995], Elmasri and Navathe [2000], O’Neil and O’Neil [2000], Ramakrishnan and Gehrke [2000], and Ullman [1988]. Textbook coverage of transaction processing is provided by Bernstein and Newcomer [1997] and Gray and Reuter [1993].

Several books contain collections of research papers on database management. Among these are Bancilhon and Buneman [1990], Date [1986], Date [1990], Kim [1995], Zaniolo et al. [1997], and Stonebraker and Hellerstein [1998].

A review of accomplishments in database management and an assessment of future research challenges appears in Silberschatz et al. [1990], Silberschatz et al. [1996] and Bernstein et al. [1998]. The home page of the ACM Special Interest Group on Management of Data (see www.acm.org/sigmod) provides a wealth of information about database research. Database vendor Web sites (see the tools section below) provide details about their respective products.

Codd [1970] is the landmark paper that introduced the relational model. Discussions concerning the evolution of DBMSs and the development of database technology are offered by Fry and Sibley [1976] and Sibley [1976].

Tools

There are a large number of commercial database systems in use today. The major ones include: IBM DB2 (www.ibm.com/software/data), Oracle (www.oracle.com), Microsoft SQL Server (www.microsoft.com/sql), Informix (www.informix.com), and Sybase (www.sybase.com). Some of these systems are available free for personal or noncommercial use, or for development, but are not free for actual deployment.

There are also a number of free/public domain database systems; widely used ones include MySQL (www.mysql.com) and PostgresSQL (www.postgressql.org).

A more complete list of links to vendor Web sites and other information is available from the home page of this book, at www.research.bell-labs.com/topic/books/db-book.

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