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Unit3rd
 

Unit3rd

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basics of computers

basics of computers

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    Unit3rd Unit3rd Document Transcript

    • UNIT III Basic concepts of databases Data Data can be defined as a representation of facts, concepts or instruction in a formalized manner suitable for communication, interpretation or processing by human or electronic machine. Data is represented with the help of characters like Alphabets (a-z, A-Z), digits (0-9) or special characters (+, -, *, /, <, >, = etc.) Database A database is a set of structured and not redundant information, whose organization is based on a data model. It consists of files, records (electronic cards) and fields. Redundancy means repetition. Different data models are discussed later. Synonyms for databases: registers, search tools, electronic card files, electronic collections. File A file (e.g. a table) consists of records. A file also contains the layout designed for these records. Record (Row) A record is a collection of information kept about one person, product or transaction. Data Item or Field (Column) Within each record information is entered into a field which represents a category of information. OR A set of character which are used together to represent a specific data element e.g. name of a student in a class is represented by the data item, say, NAME. Table A table is a database object for storing data. Each table contains information about a particular subject, such as customers. A table consists of records and fields. So, it´s a file.
    • Form A form is a database object designed primarily for data input and display. The data shown extracts from tables and queries. Database program (Database Management System, DBMS) A database management system is software which plays the role of interface between the users and the database. It provides all the data definition, data manipulation, and data control features the user needs to manage. A database management system and a database form together a database system. What is Database Management System(DBMS) A database can be termed as a repository of data. A collection of actual data which constitutes the information regarding an organization is stored in a database. For ex. There are 1000 students in a college & we have to store their personal details, marks details etc., these details will be recorded in a database. A collection of programs that enables you to store, modify, and extract information from a database is known as DBMS. The primary goal of a DBMS is to provide a way to store & retrieve database information that is both convenient & efficient. Database systems are designed to manage large bodies of information. Management of data involves both defining structures for storage of information & providing way for manipulation of data. In addition, the database system must ensure safety of data . DBMS is collection of programs that enables you to store, modify, and extract important information from a database. There are many different types of DBMS, ranging from small systems that run on personal computers to huge systems that run on mainframes.
    • Good data management is an essential prerequisite to corporate success. Provided that data is: • Complete • Accurate • Timely • Easily available Database System Applications There are many different types of DBMSs, ranging from small systems that run on personal computers to huge systems that run on mainframes. Databases are applied in wide no. of applications. Following are some of the examples:- • Banking: For customer information, accounts, loans & other banking transactions • Airlines: For reservation & schedule information • Universities: For student information, course registration, grades etc.
    • • Credit card transaction: For purchase of credit cards & generation of monthly statements. • Telecommunication: For keeping records of calls made, generating monthly billetc. • Finance: For storing information about holdings, sales & purchase of financial statements • Sales: For customer, product & purchase information • Manufacturing: For management of supply chain. • Human Resource: For recording information about employees, salaries, tax, benefits etc. We can say that when ever we need to have a computerized system, we need a database system Purpose Of Database system A file system is one in which we keep the information in operating system files. Before the evolution of DBMS, organizations used to store information in file systems. A typical file processing system is supported by a conventional operating system. The system stores permanent records in various files & it need application program to extract records, or to add or delete records . We will compare both systems with the help of an example. There is a saving bank enterprise that keeps information about all customers & saving accounts. Following manipulations has to be done with the system • A program to debit or credit an account • A program to add a new account. • A program to find balance of an account. • A program to generate monthly statements. As the need arises new applications can be added at a particular point of time as checking accounts can be added in a saving account. Using file system for storing data has got following disadvantages:- 1. Data Redundancy & Inconsistency:-
    • Different programmers work on a single project, so various files are created by different programmers at some interval of time. So various files are created in different formats & different programs are written in different programming language. Same information is repeated. For ex name & address may appear in saving account file as well as in checking account. This redundancy results in higher storage space & access cost. It also leads to data inconsistency which means that if we change some record in one place the change will not be reflected in all the places. For ex. a changed customer address may be reflected in saving record but not any where else. 2. Difficulty in Accessing data Accessing data from a list is also a difficulty in file system. Suppose we want to see the records of all customers who has a balance less than $10,000, we can either check the list & find the names manually or write an application program .If we write an application program & at some later time, we need to see the records of customer who have a balance of less than $20,000, then again a new program has to be written. It means that file processing system do not allow data to be accessed in a convenient manner. 3. Data Isolation As the data is stored in various files, & various files may be stored in different format, writing application program to retrieve the data is difficult. 4. Integrity Problems Sometimes, we need that data stored should satisfy certain constraints as in a bank a minimum deposit should be of $100. Developers enforce these constraints by writing appropriate programs but if later on some new constraint has to be added then it is difficult to change the programs to enforce them. 5. Atomicity Problems Any mechanical or electrical device is subject to failure, and so is the computer system. In this case we have to ensure that data should be restored to a consistent state. For example an amount of $50 has to be transferred from Account A to Account B. Let the amount has been debited from account A but have not been credited to Account B and in the mean time, some failure occurred. So, it will lead to an inconsistent state. So, we have to adopt a mechanism which ensures that either full transaction should be executed or no transaction should be executed i.e. the fund transfer should be atomic. 6. Concurrent access Problems Many systems allow multiple users to update the data simultaneously. It can also lead the data in an inconsistent state. Suppose a bank account contains a balance of $ 500 & two
    • customers want to withdraw $100 & $50 simultaneously. Both the transaction reads the old balance & withdraw from that old balance which will result in $450 & &400 which is incorrect. 7. Security Problems All the user of database should not be able to access all the data. For example a payroll Personnel needs to access only that part of data which has information about various employees & are not needed to access information about customer accounts. POINTS TO PONDER • A DBMS contains collection of inter-related data & collection of programs to access the data. • The primary goal of DBMS is to provide an environment that is both convenient & efficient for people to use in retrieving & storing information. • DBMS systems are ubiquitous today & most people interact either directly or indirectly with database many times every day. • Database systems are designed to store large bodies of information. • A major purpose of a DBMS is to provide users with an abstract view of data i.e. the system hides how the data is stored & maintained. VIEW OF DATA A database contains a no. of files & certain programs to access & modify these files. But the actual data is not shown to the user; the system hides actual details of how data is stored & maintained. DATA ABSTRACTION Data abstraction is the process of distilling data down to its essentials. The data when needed should be retrieved efficiently. As all the details are not of use for all the users, so we hide the actual (complex) details from users. Various level of abstraction to data is provided which are listed below:- Physical level:- It is the lowest level of abstraction & specifies how the data is actually stored. It describes the complex data structure in details.
    • Logical level:- It is the next level of abstraction & describes what data are stored in database & what relationship exists between various data. It is less complex than physical level & specifies simple structures. Though the complexity of physical level is required at logical level, but users of logical level need not know these complexities.
    • View level:- This level contains the actual data which is shown to the users. This is the highest level of abstraction & the user of this level need not know the actual details (complexity) of data storage.
    • Database Language:- As a language is required to understand any thing, similarly to create or manipulate a database we need to learn a language. Database language is divided into mainly 2 parts:- 1) DDL (Data definition language) 2) DML (Data Manipulation language) Data Definition Language (DDL) Used to specify a database scheme as a set of definitions expressed in a DDL 1. DDL statements are compiled, resulting in a set of tables stored in a special file called a data dictionary or data directory. 2. The data directory contains metadata (data about data) 3. The storage structure and access methods used by the database system are specified by a set of definitions in a special type of DDL called data storage and definition language 4. Basic idea: hide implementation details of the database schemes from the users
    • Data Manipulation Language (DML) 1. Data Manipulation is: Retrieval of information from the database Insertion of new information into the database Deletion of information in the database Modification of information in the database 2. A DML is a language which enables users to access and manipulate data. The goal is to provide efficient human interaction with the system. 3. There are two types of DML: Procedural: the user specifies what data is needed and how to get it Nonprocedural: the user only specifies what data is needed Easier for user. May not generate code as efficient as that produced by procedural languages 4. A query language is a portion of a DML involving information retrieval only. The terms DML and query language are often used synonymously. POINTS TO PONDER DBMS systems are ubiquitous today & most people interact either directly or indirectly with database many times every day. Database systems are designed to store large bodies of information. A major purpose of a DBMS is to provide users with an abstract view of data i.e. the system hides how the data is stored & maintained. Structure of a database is defined through DDL & manipulated through DML. DDL statements are compiled, resulting in a set of tables stored in a special file called a data dictionary or data directory. A query language is a portion of a DML involving information retrieval only. The terms DML and query language are often used synonymously.
    • Advantages of DBMS In DBMS, all files are integrated into one system thus reducing redundancies and making data management more efficient. In addition, DBMS provides centralized control of the operational data. Some of the advantages of data independence, integration and centralized control are: 1. Redundancies and inconsistencies can be reduced In conventional data systems, an organisation often builds a collection of application programs often created by different programmers and requiring different components of the operational data of the organisation. The data in conventional data systems is often not centralised. Some applications may require data to be combined from several systems. These several systems could well have data that is redundant as well as inconsistent (that is, different copies of the same data may have different values). Data inconsistencies are often encoutered in everyday life. For example, we have all come across situations when a new address is communicated to an organisation that we deal with (e.g. a bank, or Telecom, or a gas company), we find that some of the communications from that organisation are recived at the new address while others continue to be mailed to the old address. Combining all the data in a database would involve reduction in redundancy as well as inconsistency. It also is likely to reduce the costs for collection, storage and updating of data. 2. Better service to the Users A DBMS is often used to provide better service to the users. In conventional systems, availability of information is often poor since it normally is difficult to obtain information that the existing systems were not designed for. Once several conventional systems are combined to form one centralised data base, the availability of information and its up-to-dateness is likely to improve since the data can now be shared and the DBMS makes it easy to respond to unforseen information requests. Centralizing the data in a database also often means that users can obtain new and combined information that would have been impossible to obtain otherwise. Also, use of a DBMS should allow users that do not know programming to interact with the data more easily. The ability to quickly obtain new and combined information is becoming increasingly important in an environment where various levels of governments are requiring organisations to provide more and more information about their activities. An organisation running a conventional data processing system would
    • require new programs to be written (or the information compiled manually) to meet every new demand. 3. Flexibility of the system is improved Changes are often necessary to the contents of data stored in any system. These changes are more easily made in a database than in a conventional system in that these changes do not need to have any impact on application programs. 4. Cost of developing and maintaining systems is lower As noted earlier, it is much easier to respond to unforseen requests when the data is centralized in a database than when it is stored in conventional file systems. Although the initial cost of setting up of a database can be large, one normally expects the overall cost of setting up a database and developing and maintaining application programs to be lower than for similar service using conventional systems since the productivity of programmers can be substantially higher in using non-procedural languages that have been developed with modern DBMS than using procedural languages. 5. Standards can be enforced Since all access to the database must be through the DBMS, standards are easier to enforce. Standards may relate to the naming of the data, the format of the data, the structure of the data etc. 6. Security can be improved In conventional systems, applications are developed in an ad hoc manner. Often different system of an organisation would access different components of the operational data. In such an environment, enforcing security can be quite difficult. Setting up of a database makes it easier to enforce security restrictions since the data is now centralized. It is easier to control who has access to what parts of the database. However, setting up a database can also make it easier for a determined person to breach security. We will discuss this in the next section. 7. Integrity can be improved Since the data of the organization using a database approach is centralized and would be used by a number of users at a time, it is essential to enforce integrity controls. Integrity may be compromised in many ways. For example, someone may make a mistake in data input and the salary of a full-time employee may be input as $4,000 rather than $40,000. A student may be shown to have borrowed books but
    • has no enrolment. Salary of a staff member in one department may be coming out of the budget of another department. If a number of users are allowed to update the same data item at the same time, there is a possiblity that the result of the updates is not quite what was intended. For example, in an airline DBMS we could have a situation where the number of bookings made is larger than the capacity of the aircraft that is to be used for the flight. Controls therefore must be introduced to prevent such errors to occur because of concurrent updating activities. However, since all data is stored only once, it is often easier to maintain integrity than in conventional systems. 8. Enterprise requirements can be identified All enterprises have sections and departments and each of these units often consider the work of their unit as the most important and therefore consider their needs as the most important. Once a database has been set up with centralised control, it will be necessary to identify enterprise requirements and to balance the needs of competing units. It may become necessary to ignore some requests for information if they conflict with higher priority needs of the enterprise. 9. Data model must be developed Perhaps the most imporrant advantage of setting up a database system is the requirement that an overall data model for the enterprise be built. In conventional systems, it is more likely that files will be designed as needs of particular applications demand. The overall view is often not considered. Building an overall view of the enterprise data, although often an expensive exercise, is usually very cost-effective in the long term. Limitations of Databases In spite of the advantages of using DBS, there are certain limitations of using DBS: • Overhead Cost: When very large data need to be managed in large scale organization, then database approach is motivated. This also requires a powerful hardware platform and software for database management, which are quite expensive. Another cost incurred will be hiring of system analyst, database designers, database administrators, programmers and data processing personnels and cost of training. This means to adopt this approach, a significant extra cost has to be borne by the organization.  Security Problem:
    • Another disadvantage of this approach is that sharing of data also carries the risk of the data being accessed by unauthorized user. Thus, the organization needs to cope up this problem by taking security measures, concurrency control, recovery and integrity.  Problem of Resources: Running on-line, real time system to answer on-line queries requires large amount of data to be stored. As a result more terminals may be needed to put managers and other users online. Communication devices are also required to connect extra terminals to the database. It may require resources such as multiprocessor system and software to run a DBMS. Therefore, DBMS may require extra computing resources depending upon the application.  Ownership Problem: In file based system, programmer/user is the owner of the data and program whereas database consisting of such files is owned by entire company. For any change or read or insertion of data in the database, user needs to seek permission from managers of the company. For a database to be successful, the database must be viewed and updated as a corporate resource, not as individual resource.  Concurrency Problem: Several problems can occur when concurrent transactions execute in an uncontrolled manner. There may be lost update problem when two transactions that access the same database items have their operations inter-leaved in a way that makes the value of some database item incorrect. Entity: An entity is a person, place, thing, event or concept about which information is recorded. In a banking environment, examples of entities are CUSTOMERS, BANK ACCOUNTS etc. Attributes: Attribute gives the characteristic of the entity. In other words, every entity has some basic attributes that characterize it e.g. 1) A house can be described by its size, color, age and surroundings.
    • 2) A customer of a bank may be described by such attributes as name, address and possibly a customer identification number. 3) A bank account can be represented by an account type, an account number and an account balance. So, in example 1) size, color, age and surroundings are attributes of the entity house. In example 2) customer identification no, customer name and customer address are attributes describing the entity “CUSTOMER” of a bank. An attribute is often called a data element, a data field, a data item or an elementary item. Schema: The overall logical data-base description is referred to as a schema. It is sometimes also referred to as an overall model of the data, a conceptual model or a conceptual schema. The database schema is specified during the database design and is not expected to change frequently. A displayed schema is called a Schema Diagram. Student Name Student Number Class Major Course Course Name Course Number Credit Hours Department An example of Schema Diagram of a Database that stores Student Records. A schema diagram displays only some aspects of a schema, such as the names of record types and data items and some type of constraints. Other aspects are not specified in the schema diagram. The actual data in a database may change quite frequently e.g. in the above database the database changes every time we add a student. The data in the database at a particular moment in time is called a Database State or Snapshot. It is also called the current set of occurrences or instances in the database.