BCA-204 Directorate of Distance Education Maharshi Dayanand University ROHTAK
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1 Database Systems BCA-204 Directorate of Distance Education Maharshi Dayanand University ROHTAK
2 Copyright 2002, Maharshi Dayanand University, ROHTAK All Rights Reserved. No part of this publication may be reproduced or stored in a retrieval system or transmitted in any form or by any means; electronic, mechanical, photocopying, recording or otherwise, without the written permission of the copyright holder. Maharshi Dayanand University ROHTAK Developed & Produced by EXCEL BOOKS, A-45 Naraina, Phase 1, New Delhi
3 Contents UNIT 1 DATA MODELLING FOR A DATABASE 1 Introduction Database Benefits of the Database Approach Structure of DBMS DBA Records Files Abstraction Integration UNIT 2 DATABASE MANAGEMENT SYSTEM 28 Data Model ER Analysis Record Based Logical Model Relational Model Network Model Hierarchical Model UNIT 3 RELATIONAL DATA MANIPULATION 36 Relation Algebra Relational Calculus SQL UNIT 4 RELATIONAL DATABASE DESIGN 99 Introduction Functional Dependencies Normalisation First Normal Form Second Normal Form Third Normal Form Boyce-Codd Normal Form Fourth Normal Form Fifth Normal Form UNIT 5 QUERY PROCESSING 131
4 Introduction Query Processor General Strategies for Query Processing Query Optimization Concept of Security Concurrency Recovery UNIT 6 DATABASE DESIGN PROJECT 177 Definition and Analysis of Existing Systems Data Analysis Preliminary and Final Design Testing & Implementation Maintenance Operation and Tuning UNIT 7 USE OF RELATIONAL DBMS PACKAGE FOR CLASS PROJECT 197 Implementation of SQL using Oracle RDBMS APPENDIX 211
5 Introduction Database Benefits of the Database Approach Structure of DBMS Database Administrator Records and Record Types Files Data Integrity Constraints Data Abstraction Data Modeling for a Database Learning Objectives After reading this unit you should appreciate the following: Introduction Database Benefits of the Database Approach Structure of DBMS DBA Records Files Abstraction Integration Top The present time is known as the information age, reason being that humans are dealing with data and information related to business or organization. Since the beginning of civilization, man is manipulating data and the give and take of information has been in practice, but this has been considered as an important discipline only for the last few decades. Today, data manipulation and information processing have become the major tasks of any organization, small or big, whether it is an educational institution, government concern, scientific, commercial or any other. Thus we can say that information is the essential requirement of any business or organization. Data: It is the plural of a Greek word datum, which means any raw facts, or figure like numbers, events, letters, transactions, etc, based on which we cannot reach any conclusion. It can be useful after processing,
6 2 DATABASE SYSTEMS e.g. 78, it is simply a number (data) but if we say physics (78) then it will becomes information. It means somebody got distinctional marks in physics. Information is processed data. The user can take decision based on information. Data Processing Information Information systems, through their central role in information economy, bring about the following changes: Global exposure of the industry. Actively working people. Precedence of idea and information over money. Growth in the business size. Globalization changing technologies. Integration among different components based on information flow. Need for optimum utilization of resources. Deciding loss/benefit of business. Future oriented Information. External Interfaces. Sales Product Planning Control Corporate Database Accountin Material A/c A/c Payable Manufacturin Purchasin Requirement Schedulin Productio
7 DATA MODELLING FOR A DATABASE 3 An organization is only a mechanism for processing information and considers that the traditional management of information can be viewed in the context of information and process. The manager may be considered as a planning and decision center. Established routes of information flow are used to determine the effectiveness of the organization in achieving its objectives. Thus, information is often described as the key to success in business. Student Activity 1.1 Before reading the next section, answer the following questions: 1. Justify that numbers 90, 40, 45 are data or information. 2. What is the difference between data and information? 3. Give the example of objects by which we can judge that these are information. 4. Name a list data. 5. Make a list of information. 6. Data are the raw facts and. If your answers are correct, then proceed to the next section. A system is a group of associated activities or functions with the following attributes: A common goal or purpose Identifiable objectives A set of procedures including inputs, outputs and feedback An environment in which the system exists Mutually dependent sub systems that interact It can be understood as follows: Labour Money Goods Services and Growth Downfall or
8 4 DATABASE SYSTEMS Productivity Materials Machinery Taxes Methods Employment Processor Management Planning Organizing Staffing Controlling It is imminent from the above mentioned job profile and process of management that effective management, and effective decision making ability is directly dependent on individuals and organizational ability to manage information. Information can be collected as data or information, which some other person has introduced. Data / information can be obtained from both internal and external organizational sources. Information sources from internal and external sources may be classified as formal, or informal. The uses of formal systems are based on proper procedures of collecting data. The conversion of data into information, and the use of information, requires proper procedures to be in place. A means of identifying such sources is to look for internal systems within which inputs and outputs are impressed in a constant format. An informal internal source of information is a source of information through which management receives information outside formal procedures. Many informal sources are verbal, therefore, the organization will require procedures through which such information can be collated for future use. Informal and formal information may also be obtained from external sources like newspapers, To etc. In addition to identifying potential sources the organization will need to devise systems through which internal information can be collated for potential future use. The basic quality of good information is that it has some value for the recipient. A measure of value can be found in its usefulness and reliability to the recipient. The value of the information will be decreased if the level of inaccuracy is unknown or the cost of compensating for the inaccuracies are greater than the potential benefits. To have value, the information must be used. The value an organization gains from information, relates to the decision making process and the ability of the management to improve profitability through use of information.
9 DATA MODELLING FOR A DATABASE 5 Information should be provided at all levels. The objective of the provision of information is to enable managers to control those areas, which they have responsibility for. Information will come from internal and external sources and has to be communicated on time to facilitate effective decision-making. Management is another form of the system, which comprises of elements and/or activities, which relate to the achievement of a goal. Management control: is the means through which managers ensure that required resources are obtained and used effectively and efficiently to accomplish the objectives of the organization. Operational control: Ensures that specific tasks are undertaken and completed effectively and efficiently. Operational control has become less important with automation because tasks are increasingly becoming subject to programmed control. Strategic information: Enables directors and senior managers to plan the organization s overall objectives and strategies. Tactical information: Used at all management levels, but mainly at middle management level, for tactical planning and managing control function. Operational information: The managers at the operational level who need to ensure that routine tasks are being correctly planned and controlled use this information. These decisions are usually highly structured and repetitive [i.e. programmable] with the required information being presented on a regular basis. Collecting data to provide information is a time-consuming exercise. The developer must check that the extra information gained is worthwhile, both in terms of cost and time. The assessment of what is valuable data is carried out before any data is collected. Clear objectives of the intended system should be used to determine data requirements. The company may commission a market research survey to analyses customer-buying habits. The range of investigation and size of the survey sample would be controlled by the survey budge. We expect information to be reliable and accurate. These features can be measured by the degree of completeness, precision and timeliness. The user of information should receive all the details necessary to aid decision-making. It is important for all information to be supplied before decisions are made. For example, new stock should not be ordered until full details of current stock levels are known. This is a simple example, since we know what information is required and where to obtain it. Difficulties begin when we are not sure of the completeness of the information received. Business analysts and economic advisors are well aware of these problems when devising strategies and fiscal plans. Inaccurate information can be more damaging than incomplete information to a business. The degree of accuracy required depends on the recipient s position in the management hierarchy. In general terms, the higher the position, the less accuracy required. Decisions made at the top management level are based on
10 6 DATABASE SYSTEMS annual summaries of items such as sales, purchases and capital spending. Middle managers would require a greater degree of accuracy, perhaps weekly or monthly totals. Junior management requires the greatest degree of accuracy to aid decision-making. Daily up-to-date information is often necessary, with accuracy to the nearest percentage point or unit. This is described as the provision of prepared information as soon as it is required. We also need to consider the case where accurate information is produced, but not used immediately, rendering it out-ofdate. Some systems demand timely information and cannot operate without it. Airline reservation systems are one example, passengers and airline staff depend on timely information concerning flight times, reservations and hold-ups. Student Activity 1.2 Before reading the next section, answer the following questions: 1. What is source? 2. Define source s types. 3. What are various qualities of a good information? 4. Information will come from.. and.. 5. The basic quality of good information is that it has some value for the.. 6. Give the examples of source. If your answers are correct, then proceed to the next section. This is a traditional term used to describe the processing of function-related data with a business organization. Sales order processing is a typical example of data processing. Note that processing may be carried out manually or using a computer. Some systems employ a combination of both manual and computerized processing techniques. In both the cases, the data processing is essentially. Differences can be described in terms of: Computers can process data much quicker than any human. Hence, a computer system has a potentially higher level of productivity and, therefore, it is cheaper for high-volume data processing. Speed allows more timely information to be generated. Computers have a reputation for accuracy, assuming that correct data has been input and that procedures define processing steps correctly. The errors in computer systems are thus human errors (software, or input), or less likely, machine errors (hardware failure).
11 DATA MODELLING FOR A DATABASE 7 As processing requirements increase, possibly due to business expansion, manages require more information processing. Human systems cannot cope up with these demands. Banking is a prime example where the dependency on computers is total. There are some tasks that computers cannot perform. These activities usually have a high degree of nonprocedural thinking in which the rules of processing are difficult to define - it would be extremely difficult to produce a set of rules even for safety in crossing a busy road. Many management posts still rely to a great degree on human decision-making. Top management decisions on policy and future business are still determined by a board of directors and not by a computer. Having understood the basic concept and significance of information and database, let us now get into the basics: Data: As we described earlier, Data are the raw facts used for information processing. Data must be collected and then input ready for processing. Each item of data must be clearly labeled, formatted and its size determined. For example, a customer account number may be labeled A/C, in numeric format, of size of five digits. Data may enter a system in one form and then be changed as it is processed or calculated. Customer order data, for example, may be converted to electronic form by keying in the orders from specially prepared data entry forms. The order data may then be used to update both customer and stock files. Input: The transaction is the primary data input which leads to system action, e.g., the input of a customer order to the sales order processing system. The volume and frequency of transactions will often determine the structure of an organization. In addition to transaction data, a business system will also need reference to stored data, know as standing or fixed data. Within a sales order processing system we have standing data in the form of customer names and addresses, stock records and price lists. The transactions contain some standing data, for referencing, but mainly variable data, such as items and quantities ordered. Output: Output from a business system is often seen as planning or control information, or as input to another system. This can be understood if we consider a stock control system. Output will be stock level information, slow-and fast-moving items for example are stock orders, for items whose quantities fall below their reorder level. Stock movement information would be used to plan stock levels and reorder levels, whilst stock order requirements would be used as input to the purchasing system. Files: A file is an ordered collection of data records, stored for retrieval or amendment, as required. When files are amended from transaction data, this is referred to as updating. In order to aid information flow, files may be shared between sub systems. For example, a stock file may be shared between the sales function and the purchasing function. Processes: Data is converted to output or information by processing. Processing examples include sorting, calculating and extracting. Student Activity 1.3 Before reading the next section, answer the following questions:
12 8 DATABASE SYSTEMS 1. Why we process the data? 2. What is input? 3. What is output? 4. Is data processing essential? 5. Speed, accuracy, volume are used in favour of. 6. Output from a business system is often seen as. If your answers are correct, then proceed to the next section. Top A database is a collection of related data or operational data extracted from any firm or organization. For example, consider the names, telephone number, and address of people you know. You may have recorded this data in an indexed address book, or you may have stored it on a diskette, using a personal computer and software such as Microsoft Access of MS Office or ORACLE, SQL SERVER etc. The common use of the term database is usually more restricted. A database has the following implicit properties: A database represents some aspect of the real world, sometimes called the miniworld or the Universe of Discourse (U.D.). Changes to the miniworld are reflected in the database. A database is a logically coherent collection of data with some inherent meaning. A random assortment of data cannot correctly be referred to as a database. A database is designed, built and populated with data for a specific purpose. It has an intended group of users and some preconceived applications in which these users are interested. In other words, a database has some source from which data is derived, some degree of interaction with events and an audience that is actively interested in the contents of the database. A database can be of any size and of varying complexity. For example, the list of names and addresses referred to earlier may consist of only a few hundred records, each with a simple structure. On the other hand, the card catalog of a large library may contain half a million cards stored under different categories by primary author s last name, by subject, by book titles with each category organized in alphabetic order. Here are several examples of databases. 1. Manufacturing company 2. Bank 3. Hospital 4. University 5. Government department In general, it is a collection of files (tables) Entity: A person, place, thing or event about which information must be kept. Attribute: Pieces of information describing a particular entity. These are mainly the characteristics about the individual entity. Individual attributes help to identify and distinguish one entity from another.
13 DATA MODELLING FOR A DATABASE 9 # $ %%%& % '%(%) '!" Bit 0,1 Byte (8-bits) Field (Attribute name like name, Age, Address) Record (One or more rows in a table) File (Table or collection of all files) Database (Collection of files or tables) e.g. Student (Database Name)!" #" $ () ** ( + )'! ( (!, -( ( Handling of a small shop s database can be done normally but if you have a large database and multiple users then in that case you have to maintain computerized database. The advantages of a database system over traditional, paper-based methods of record-keeping tag will perhaps be more readily apparent in these examples. Here are some of them. Compactness: No need for possibly voluminous paper files. Speed: The machine can retrieve and change data faster than a human can. Less drudgrey: Much of the sheer tedium of maintaining files by hand is eliminated. Mechanical tasks are always better done by machines.
14 10 DATABASE SYSTEMS Currency: Accurate, up-to-date information is available on demand at any time. Top % " && " There are following benefits of the Database Approach: Redundancy and duplication can be reduced. In the database approach, the views of different user groups are integrated during database design. For consistency, we should have a database design that stores each logical data item such as student s name or birth date in only one place in the database. This does not permit inconsistency, and it saves. However, in some cases, controlled redundancy may be useful for improving the performance of queries. For example, we may store Student Name and Course Number redundantly in a GRADE_REPORT file(fig below), because whenever we retrieve a GRADE_REPORT record, we want to retrieve the student name and course number along with the grade, student number, and section identifier. By placing all the data together, we do not have to search multiple files to collect this data.! "#$%! "#$ & Inconsistency can be avoided (to some extent). Employee E4 works in department D5 is represented by two distinct entries in the stored database. Suppose also that the DBMS is not aware of this duplication (i.e. redundancy is not controlled). Then there will necessarily be an occasion on which the two entries will not agree, i.e., when one of the two has been epilated and the other has not. At such times the database is said to be inconsistent. The data can be shared. Same database can be used by variety of users, for their different objectives, simultaneously. Security restrictions can be applied. It is likely that some users is often will not be authorized to access all information in the database. For example, financial data is often considered confidential, and hence only authorized persons are allowed to access such data. In addition, some users may be permitted only to retrieve data, whereas others are allowed both to retrieve and to-update. Integrity can be maintained. The problem of integrity is the problem of ensuring that the data in the database in accentuate it means if the data type of any field is number then we cannot insert any string text here. Student Activity 1.4
15 DATA MODELLING FOR A DATABASE 11 Before reading the next section, answer the following questions: 1. What is database? 2. What is record? 3. What is field? 4. Why database is needed? 5. What is redundancy? 6. What are the benefits of database? If your answers are correct, then proceed to the next section. #" $ A DBMS is a sophisticated piece of software, which supports the creation, manipulation and administration of database system. A database system comprises a database of operational data together with the processing functionality required to access and manage that data. Typically, this means a computerized record keeping system whose overall purpose is to maintain information and to make that information available on demand. ).- /. 0. ).- $$*(. $/ + $$ This picture shows a greatly simplified view of a database system. The figure is intended to illustrate the point that a database system involves four major components namely, data, hardware, software, and users. " % ' &" () The DBMS responds to a query by invoking the appropriate sub-programmes, each of which performs its special function to interpret the query, or to locate the desired data in the database and insert it in the designed order. Thus DBMS shields database users from the tedious programming they would have to do, organize data for storage, or to gain access to it once it has been stored. +1
16 $3 12 DATABASE SYSTEMS 2 # * 3 ) # # ) As already mentioned, a database consists of a group of related files of different record types and the DBMS allows users to access data anywhere in the database, without the knowledge of how data are actually organized on the storage device. Student Activity 1.5 Before reading the next section, answer the following questions: 1. Define DBMS. 2. Why DBMS is needed? 3. Users requests are handled by 4. A database consists of.. 5. DBMS is an interface between database and.. 6. Give the examples of several DBMS. If your answers are correct, then proceed to the next section. Top % The role of the DBMS as an intermediary between the users and the database is very much like the function of a salesperson in a consumer distributor system. A consumer specifies desired items by filling out an order form, which is submitted to a salesperson at the counter. The salesperson presents the specified items to consumer after they have been retrieved from the storage room. Consumers who place orders have no idea of where and how the items are stored; they simply select the desired items from an alphabetical list in a catalogue. However, the logical order of goods in the catalogue bears no relationship to the actual physical arrangement of the inventory in the storage room. Similarly, the database user needs to know only what data he or she requires; the DBMS will take care of retrieving it. Database Management Systems: A database management system (DBMS) is a software application system that is used to create, maintain and provide controlled access to user databases. Database management systems range in complexity from a PC-DBMS (such as Ashton Tate s dbase IV) costing a few hundred dollars to a mainframe DBMS product (such as IBM s DB2) costing several hundred thousand dollars. The major components of a full-function DBMS are shown in the diagram given below: * ) #
17 DATA MODELLING FOR A DATABASE 13 *+%, ' () The engine is the central component of a DBMS. This module provides access to the reposition and the database and coordinates all the other functional elements of the DBMS. The DBMS engine receives logical requests for data (and metadata) from human users and from applications, determines the secondary storage location of those data and issues physical input/output requests to the computer operating system. The engine provides services such as memory and buffer management, maintenance of indexes and lists and secondary storage or disk management. *-+ The interface sub system provides for users and applications to access the various components of the DBMS. Most DBMS products provide a range of languages and other interfaces, since programmers (or other technical persons by users with little or no programming experience will use the system. Some of the typical interfaces to a DBMS are the following: A data definition language (or data sub-language), which is used to define database structures such as records, tables, files and vies. An interactive query language (such as SQL), which is used to display data extracted from the database and to perform simple updates. A graphic interface (such as Query-by-Example) in which the system displays a skeleton table (or tables) and users propose requests by suitable entries in the table. A forms interface in which a screen oriented form is presented to the user, who responds by filling in blanks in the form. A DBMS programming language, (such as the DBMS IV command language), which is procedural language that allows programmers to develop sophisticated application. An interface to standard third-generation programming languages such as BASIC and COBOL. A natural language interface that allows users to present requests in free-form English statements.
18 14 DATABASE SYSTEMS *.+ (& The information repository dictionary sub system is used to manage and control access to the repository. IRDS is a component that is integrated within the DBMS. Notice that the IRDS uses the facilities of the database engine to manage the repository. */+ The performance management sub system provides facilities to optimize (or at least improve) DBMS performance. Two of its important functions follow: Query optimization. Structuring SQL queries (or other forms of user queries) to minimize response times. *0+ The data integrity management sub system provides facilities for managing the integrity of Data in the database and the integrity of metadata in the repository. There are three important functions: 1. Intra-record integrity: Enforcing constraints or data item values and types within each record in the database. 2. Referential integrity: Enforcing the validity of references between records in the database. 3. Concurrency control: Assuring the validity of database updates when multiple users access the database (discussed in al later section). *1+%&( 2 The backup and recovery sub system provides facilities for logging transactions and database changes, periodically making backup copies of the database and recovering the database in the event of some type of failure. *3+&& 2 & The application development sub system provides facilities that allow end users and/or programmers to develop complete database applications. It includes CASE tools as well as facilities such as screen generators and report generators. *4+ The security management sub system provides facilities to protect and control access to the database and repository. What has been described above is the manifestation of individual components of a typical DBMS; we would again look at these components from another view later in this section. But first, it is relatively important to focus on some of the other interfacing aspects of DBMS software. What might be termed the third-generation approach to systems development involved the production of a suite of programmers that together constituted an application system- a self-contained functional capability to do something useful. Within an application system, each program manipulates data in one for more files and a particular file might be both read and written by several programmers. An organization typically develops several application systems for each-one information systems task perceived. The DBMS (database approach) tries to overcome all of the shortcomings of the pre database approach as follows:
19 DATA MODELLING FOR A DATABASE 15 Data Validation Problems: If many programs manipulate a particular type of information then validation of its correctness must be carried out by each of those on guard against entry of any illegal values. Consequently, program code may need to duplicate and, if the validation conditions change, each program (at least) must be recompiled. Data Sharing Problems: Perhaps more seriously, if a file is used by several programmes and there is a need to change its structures in some way, perhaps to add a new type information object that is required by a new program, then each program will need to be recompiled-unless one maintains duplicate information in different structures, in which case there is a synchronization problem. A further dimension to this problem results from the fact that with conventional operating system facilities, if two or more programs write to the same file at the same time unpredictable results will be obtained. Concurrent update must be avoided either by use imposed synchronization (that is, manually controlling the usage of programmers), or by locking scheme that would have to be implemented by the application programs. In either case, there are costs-management control or programming effort. Manipulation Problems: When writing a program using a conventional programming language and operating system facilities, a programmer uses record-level commands (i.e. reads and writes) on each file to perform the required functions; this is laborious and hence unproductive of the programmer s time. Data Redundancy: The same piece of information may be stored in two or more files. For example, the particulars of an individual who may be a customer and an employee may be stored in two or more files. Program/Data Dependency: In the traditional approach, if a data field is to be added to a master file, all such programmes that access the master file would have to be changed to allow for this new field which would have been added to the master record. Lack of Flexibility: In view of the strong coupling between the programme and the data, most information retrieval possibilities would be limited to well-anticipated and predetermined requests for data, the system would normally be capable of producing schedule records and queries which it would have been programmed to create. In the fast moving and competent business environment of today, apart from such regularly scheduled records there is a need for responding to un-anticipatory queries and some kind of investigative analysis which cannot be envisaged professionally. One could discuss other points and security problems which would probably come next, but the above should be sufficient to illustrate that the approach is fundamentally inadequate for the problem to which it has been applied. Agreement in this matter has grown since the mid-1960s and the database approach is now well established as the basis for information system development and management in many application areas. All the above difficulties result from two surroundings: The lack of any definition of data objects independently of the way in which they are used by specific application programmes; and The lack of control over data object manipulation beyond that imposed by existing application programmers. The database approach has emerged in response. Fundamentally it rests on the following two interrelated ideas: The extraction of data object type descriptions from application programmes into a single repository called a database schema (the word schema can be taken to mean a description of form)-an application-independent description of the object in the database; and
20 16 DATABASE SYSTEMS The introduction of a software component called a database management system (DBMS) through which all data definition and manipulation (update and interrogation) occurs-a buffer that controls data access and removes this function from the applications. Together, these ideas have the effect of fixing a funnel over the top of the data used by application systems and forcing all application program s data manipulation through it. So let us now try to appreciate how DBMS solves some of the issues. Data Validation: In principle, validation rules for data objects can be held in the schema and enforced on entry by the DBMS. This reduces the amount of application code that is needed. Changes to these rules need be made exactly once because they are not duplicated. Data Sharing: Changes to the structures of data objects are registered by modifications to the schema. Existing application programmes need not be aware of any differences, because a correspondence between their view of data and that, which is now supported, can also be held in the schema and interpreted by the DBMS. This concept is often referred to as data independence; applications are independent of the actual representation of their data. Synchronization of concurrent access can be implemented by the DBMS because it oversees all database access. The record-level data manipulation concept of programming languages such as Cobol, PL/1, Fortran and so on can be escaped by means of a higher-level (more problem-oriented than implementation-oriented) data manipulation language that is embedded within application programs can be improved. Furthermore, because the approach involves a central repository of data description, it is possible to develop a mechanism that provides a general inquiry facility to data objects and their descriptions; such a mechanism is normally called a query language. It is interesting that the emergence of the database approach has brought about a new class of programming language; this is symptomatic of the significant change that database thinking has brought to the information systems development process. Having described the database approach in terms of its impact on the development and management of information systems, it is now appropriate to attempt some definitions. Student Activity 1.6 Before reading the next section, answer the following questions: 1. Make a diagram showing the structure of DBMS. 2. What are the major components of DBMS? 3. What is DBMS engine? 4. Write brief notes on security management sub system. 5. What are the functions of data integrity? 6. What do you understand by data validation? If your answers are correct, then proceed to the next section. Top
21 DATA MODELLING FOR A DATABASE 17 One of the main reasons for using DBMS is to have central control of both the data and the processes that access those data. The person who has such central control over the system is called the database administrator (DBA). The functions of the DBA include the following: Schema Definition: The DBA creates the original database schema by writing a set of defines that is translated by the DDL (Data Def n. Lang.) Compiler to a set of tables that is store permanently in the data dictionary. Storage Structure and Access-Method Definition: The DBA creates appropriate storage structures and access methods by writing a set of definitions, which is translated by the DDL compiler. Schema and Physical-Organization Modification: Programmers accomplish the relatively rare modifications either to the database schema or to the description of the physical storage organization. By writing a set of definitions that is used by either the DDL compiler or the data-storage and data def n. Language compilers to generate modifications to the appropriate intend system-tables (for example, the data dictionary). Growing of Authorizations for Data Access: The granting of different types of authorizations allows the DBA to regulate the parts of the database, which various users can access. Integrity-Constraint Specification: The data values stored in the database must satisfy certain consistency constraints e.g., perhaps the number of hours an employee may work in 1 week may not exceed a pre-specified limit (say 80 hours) The primary goal of a database system is to provide an environment for retrieving information from and storing new information into the database. There are four different types of database system users, differentiated by the way that they expect to interact with the system. Application programmers are computer professionals who interact with the system through DML (Data Manipulation Language) calls, which are embedded in a program written in a host language (for example, Cobol, PL/S, Pascal, C). These programs are commonly referred as application programs. e.g.: A Banking system includes programs that generate payroll checks that debit accounts, that credit accounts, or that transfer funds between accounts. Sophisticated Users: Such users interact with the system without writing programs. Instead, they form their requests in database query language. Each such query is submitted to a very processor whose function is to breakdown DML statement into instructions that the storage manager understands. Analysts who submit to explore data in the database till in the category. Specialized Users: Such users are those who write specialized database applications that do not fit into the fractional data-processing framework. e.g. computer-aided design systems, knowledge base and expert systems, systems that store data with complex data types (for example, graphics data and audio data). Naive users: These users are unsophisticated who interact with the system by involving one of the permanent application programs that have been written. For example, a bank teller who needs to transfer $50 from account A to account B invokes a program called transfer. This program asks the teller for the amount of money to be transferred, the account from which the money is to be transferred, and the account to which the money is to be transferred. "
22 18 DATABASE SYSTEMS Database changes over time when and as information is inserted and deleted. 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, schemas one changes infrequently, if at all. Analogies to the concepts of data types, variables and values in programming languages are useful here. Returning to the customer record types definition, note that in declaring the type of customer, we have not declared any variables. To declare such variables in a Pascal-like language, we write Var customer: customer; variable customer2 now corresponds to an area of storage containing a customer type record. A database schema corresponds to the programming-language type definition. A variable of a given type has a particular value at a given instant. Thus, the value of a variable in programming languages corresponds to an instance of a database schema. In other words the description of a database is called the database schema, which is specified during database design and is not expected to change frequently, A displayed schema is called a schema diagram. E.g. student-schema. ' ( # (' ( ) A schema diagram displays only some aspects of a schema, such as the names of record types and data items, and some types of constraints. Other aspects are not specified in the schema diagram. As in the above diagram they re neither in data type of each data item, nor in the relationships among the various files. Student Activity 1.7 Before reading the next section, answer the following questions: 1. What do you understand by DBA and how he plays an important role? 2. Describe various types of Database users? 3. Differentiate between instances and schemes? If your answers are correct, then proceed to the next section. Top ( ( & Data is usually stored in the form of records. Each record consists of a collection of related data values or items where each value is formed of one or more bytes and corresponds to a particular field of the record. Records usually describe entities and their attributes. For example, an EMPLOYEE and record represents an employee entity, and each field value in the record specifies some attribute of that employee, such as NAME, BIRTHDATE, SALARY, or SUPERVISOR. A collection of field names and their corresponding data types constitutes a record type or record format definition. A data type, associated with each field, specifies the type of values a field can take.
23 DATA MODELLING FOR A DATABASE 19 The data type of a field is usually one of the standard data types used in programming. These include numeric (integer, long integer, or floating point), string of characters (fixed-length or varying), Boolean (having 0 and 1 or TRUE and FALSE values only), and sometimes specially coded data and time data types. The number of bytes required for each data type is fixed for a given computer system. An integer may require 4 bytes, a long integer 8 bytes, a real number 4 bytes, a Boolean 1 byte, a Boolean 1 byte, a date 10 bytes (assuming a format of YYYY-MM-DD), and a fixed-length string of k characters k bytes. Variable-length strings may require, as many bytes as there are characters in each field value. For example, an EMPLOYEE record type may be defined using the C programming language notation as the following structure: Struct employee{ char name [30]; char ssn[9]; int salary; int jobcode; char department[20]; }; In recent database applications, the need may arise for storing data items that consist of large unstructured objects, which represent images, digitized video or audio streams, or free text. These are referred to as BLOBs (Binary Large Objects). A BLOB data item is typically stored separately from its record in a pool in a pool of disk blocks, and a pointer to the BLOB is included in the record. Top 5 A file is a sequence of records. In many cases, all records in a file are of the same record type. If every record in the file has exactly the same size (in bytes), the file is said to be made up of fixed-length records. If different records in the file have different sizes, the file is said to be made up of variable-length records. A file may have variable-length records for several reasons: The file records are of the same record type, but one or more of the field are of varying size (variable-length fields). For example, the NAME field of EMPLOYEE can be a variable-length field. The file records are of the same record type, but one or more of the field may have multiple values for individual records; such a field is called a repeating field and a group of values for the field is often called a repeating group. The file records are of the same record type, but one or more of the fields are optional; that is, they may have values for some but not all of the file records (optional fields). The file contains records of different record types and hence of varying size (mixed file). This would occur if related records of different types were clustered (placed together) on disk blocks; for example, the GRADE_REPORT records of a particular student may be placed following that STUDENT s record. The fixed-length EMPLOYEE records in the Figure given below have a record size of 71 bytes. Every record has the same fields, and field lengths are fixed, so the system can identify the starting byte position of each field relative to the starting position of the record. This facilitates locating field values by programs
24 20 DATABASE SYSTEMS that access such files. Notice that it is possible to represent a file that logically should have variable-length records as a fixed-length records file. For example, in the case of optional fields we could have each field in every file record but store a special null value of no value exists for that field. For a repeating field, we could allocate as many spaces in each record as the maximum number of values that the field can take. In either case, space is wasted when certain records do not have values for all the physical spaces provided in each record. We now consider other options for formatting records of a file of variable-length records. *+, & + -.* && ', +, * ',& $$* For variable-length fields; each record has a value for each field, but we do not know the exact length of some field values. To determine the bytes within a particular record that represent each field, we can use special separator characters (such as? or % or $) which do not appear in any field value to terminate variable-length fields (See Figure), or we can store the length in bytes of the field in the record, preceding the field value. A file of records with optional fields can be formatted in different ways. If the total number of fields for the record type is large but the number of fields that actually appear in a typical record is small, we can include in each record a sequence of <field-name, field-value>pairs rather than just the field values. Three types of separator character for the first two purposes separating the field name from the field value and separating one field from the next field. A more practical option is to assign a short field type code say, an integer number to each field and include in each record a sequence of <field-type, field-value> pairs rather than <field-name, field-value> pairs. A repeating field needs one separator character to separate the repeating values of the field and another separator character to indicate termination of the field. Finally, for a file that includes records of different types, each record is preceded by a record type indicator. Understandably, programs that process files of variable-length records which are usually part of the file system and hence hidden from the typical
25 DATA MODELLING FOR A DATABASE 21 programmers need to be more complex than those for fixed-length records, where the starting position and size of each field are known and fixed. Top 6 Most database applications have certain integrity constraints that must hold for the data. A DBMS should provide capabilities for defining and enforcing these constraints. The simplest type of integrity constraint involves specifying a data type for each data item. For example, in Figure we may specify that the value of the Class data item within each student record must be an integer between 1 and 5 and that the value of Name must be a string of no more than 30 alphabetic characters. A more complex type of constraint that occurs frequently involves specifying that a record in one file must be related to records in other files. For example, in Figure given below, we can specify that every section record must be related to a course record. Another type of constraint specifies uniqueness on data item values, such as every course record must have a unique value for Course Number. These constraints are derived from the meaning or semantics of the data and of the miniword it represents. It is the database designer s responsibility to identify integrity constraints during database design. Some constraints can be specified to the DBMS and automatically enforced. Other constraints may have to be checked by update programs or at the time of data entry. A data item may be entered erroneously and still satisfy the specified integrity constraints. For example, if a student receives a grade of A but a grade of C is entered in the database, the DBMS cannot discover this error automatically, because C is a valid value for accounts. Since application programs are added to the system in an ad hoc manner, it is difficult to enforce such security constraints. ) /!4! ( -5 - ( 0 $ ( (!6!7 ( ) ( 667 ( ).8!7 6.8 ) ( 66"7 6 ( 1 ",.8!7 9" : 9 (!6!7 9"!7 ( :!!.8!7 99 (!!9 (!6!7 99!6, ( 66"7 99
26 22 DATABASE SYSTEMS!,#$!!4!! -!4!!9 ( " ", " 9 "!7 - "!6, 2 2 ( 66"7 (,667 ( 66"7.8!7 ( 667 (!6!7 Top For the system to be usable, it must retrieve data efficiently. This concern has led to the design of complex data structures for the representation of 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 systems: Physical level. The lowest level of abstraction describes how the data are actually stored. At the physical level, complex low-level data structures are described in detail. Logical level. The next higher level of abstraction describes what data are stored in the database, and what relationships exist among those data. The entire database is thus described in terms of a small number of relatively simple structures. Although implementation of the simple structures at the logical level may involve complex physical-level structures, the user of the logical level does not need to be aware of this complexity. Database administrators, who must decide what information is to be kept in the database, use the logical level of abstraction. View level. The highest level of abstraction describes only part of the entire database. Despite the use of simpler structures at the logical level, some complexity remains, because of the large size of the database. Many users of the database system will not be concerned with all this information. Instead, such users need to access only a part of the database. So that their interaction with the system is simplified, the view level of abstraction is defined. The system may provide many views for the same database. The interrelationship among these three levels of abstraction is illustrated in Figure given below.
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