DATABASES

 

       DATABASES:-

What Is a Database?

 A database can be summarily described as a repository for data. This makes clear that building databases is really a continuation of a human activity that has existed since writing began; it can be applied to the result of any bookkeeping or recording activity that occurred long before the advent of the computer era. However, this description is too vague for some of our purposes, and we refine it as we go along. The creation of a database is required by the operation of an enterprise. We use the term enterprise to designate a variety of endeavors that range from an airline, a bank, or a manufacturing company to a stamp collection or keeping track of people to whom you want to write New Year cards. Throughout this book we use a running example that deals with the database of a small college. The college keeps track of its students, its instructors, the courses taught by the college, grades received by students, and the assignment of advisors to students, as well as other aspects of the activity of the institution that we discuss later. These data items constitute the operational data — that is, the data that the college needs to function. Operational data are built from various input data (application forms for students, registration forms, grade lists, schedules) and is used for generating output data (transcripts, registration records, administrative reports, etc.) Note that no computer is necessary for 

Why Use a DBMS? 

❖ Data independence and efficient access. 

❖ Reduced application development time. 

❖ Data integrity and security.

 ❖ Uniform data administration. 

❖ Concurrent access, recovery from crashes. 

Why Study Databases??

 ❖ Shift from computation to information – at the “low end”: scramble to webspace (a mess!) – at the “high end”: scientific applications

 ❖ Datasets increasing in diversity and volume. – Digital libraries, interactive video, Human Genome project, EOS project – ... need for DBMS exploding 

❖ DBMS encompasses most of CS – OS, languages, theory, “A”I, multimedia, logic.

Data Models 

❖ A data model is a collection of concepts for describing data.

 ❖ A schema is a description of a particular collection of data, using the a given data model. 

❖ The relational model of data is the most widely used model today. – Main concept: relation, basically a table with rows and columns. – Every relation has a schema, which describes the columns, or fields

Database System Software

 Users interact with database systems through query languages. The query language of a DBMS has two broad tasks: to define the data structures that serve as receptacles for the data of the database, and to allow the speedy retrieval and modification of data. Accordingly, we distinguish between two components of a query language: the data definition component and the data manipulation component. The main tasks of data manipulation are data retrieval and data update. Data retrieval entails obtaining data stored in the database that satisfies a certain specification formulated by the user in a query. Data updates include data modification, deletion and insertion. Programming in query languages of DBMSs is done differently from programming in higher-level programming languages. The typical program written in C, Pascal, or PL/1 directly implements an algorithm for solving a problem. A query written in a database query language merely states what the problem is and leaves the construction of the code that solves the problem to a special component of the DBMS software. This approach to programming is called nonprocedural. A central task of DBMSs is transaction management. A transaction is a sequence of database operations (that usually consists of updates, with possible retrievals) that must be executed in its entirety or not at all. This property of transactions is known as atomicity. A typical example includes the transfer of funds between two account records A and B in the database of a bank. Such a banking operation should not modify the total amount of funds that the bank has in its accounts, which is a clear consistency requirement for the database. The transaction consists of the following sequence of operations: 

1. Decrease the balance of account A by d dollars; 

2. Increase the balance of account B by d dollars. 

A Historical Perspective of Database Systems 

The history of DBMSs begins in the late 1960s, when an IBM product named IMS (Information Management System) was launched. Data was structured hierarchically, in forests of trees of records, providing very fast access. A few years after IMS appeared, in 1971, the CODASYL Database Task Group proposed a new type of database models known today as the network model. The original report considered DBMSs as extensions of the COBOL language, and structured data contained by databases as graphs of records, consisting essentially of circular linked lists. The origins of the relational model, that is the mainstay of contemporary databases are in E. F. Codd’s work in the early and mid 1970s. The development of relational database began in the late 1970s and early 1980s with an experimental relational database sytem at IBM called System R, a precursor of commercial IBM DBMSs, SQL/DS and DB2. A multitude of DBMSs emerged in the 1980s, such as ORACLE, INGRES, Rdb, etc. Relational technology evolved further in the 1990s with the addition of ideas and techniques inspired by object-oriented programming. 

 Bibliographical Comments

 Codd’s foundational work in relational databases is included in several articles [Codd, 1970; Codd, 1972a; Codd, 1972b; Codd, 1974], and [Codd, 1990]. Standard references in the database literature that contain extensive bibliographies are [Date, 2003; Elmasri and Navathe, 2006; Silberschatz et al., 2005] and [Ullman, 1988a; Ullman, 1988b]. 

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