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Unit – I Database System Concepts & Architecture
A Database Management System allows a person or user to organize, store, and retrieve data from a computer. A database, as a collection of information, can be organized so a Database Management System can access and pull specific information. A DBMS is a software that allows creation, definition and manipulation of database, allowing users to store, process and analyse data easily. DBMS provides us with an interface or a tool, to perform various operations like creating database, storing data in it, updating data, creating tables in the database and a lot more. DBMS also provides protection and security to the databases. It also maintains data consistency in case of multiple users. Some examples of popular DBMS used these days: MySql Oracle SQL Server IBM DB PostgreSQL Amazon SimpleDB (cloud based) etc. Applications where we use Database Management Systems are:
Telecom : There is a database to keeps track of the information regarding calls made, network usage, customer details etc. Without the database systems it is hard to maintain that huge amount of data that keeps updating every millisecond. Industry : Where it is a manufacturing unit, warehouse or distribution centre, each one needs a database to keep the records of ins and outs. For example distribution centre should keep a track of the product units that supplied into the centre as well as the products that got delivered out from the distribution centre on each day; this is where DBMS comes into picture. Banking System : For storing customer info, tracking day to day credit and debit transactions, generating bank statements etc. All this work has been done with the help of Database management systems. Education sector : Database systems are frequently used in schools and colleges to store and retrieve the data regarding student details, staff details, course details, exam details, payroll data, attendance details, fees details etc. There is a hell lot amount of inter-related data that needs to be stored and retrieved in an efficient manner. Online shopping : You must be aware of the online shopping websites such as Amazon, Flipkart etc. These sites store the product information, your addresses and preferences, credit details and provide you the relevant list of products based on your query. All this involves a Database management system.
Characteristics of Database Management System A database management system has following characteristics:
- Data stored into Tables: Data is never directly stored into the database. Data is stored into tables, created inside the database. DBMS also allows to have relationships between tables which makes the data more meaningful and connected. You can easily understand what type of data is stored where by looking at all the tables created in a database.
- Reduced Redundancy: In the modern world hard drives are very cheap, but earlier when hard drives were too expensive, unnecessary repetition of data in database was a big problem. But DBMS follows Normalisation which divides the data in such a way that repetition is minimum.
- Data Consistency: On Live data, i.e. data that is being continuosly updated and added, maintaining the consistency of data can become a challenge. But DBMS handles it all by itself.
- Support Multiple user and Concurrent Access: DBMS allows multiple users to work on it(update, insert, delete data) at the same time and still manages to maintain the data consistency.
- Query Language: DBMS provides users with a simple Query language, using which data can be easily fetched, inserted, deleted and updated in a database.
- Security: The DBMS also takes care of the security of data, protecting the data from un- authorised access. In a typical DBMS, we can create user accounts with different access permissions, using which we can easily secure our data by restricting user access.
- DBMS supports transactions , which allows us to better handle and manage data integrity in real world applications where multi-threading is extensively used.
Advantages of Database Management System (DBMS)
1. Improved data sharing An advantage of the database management approach is, the DBMS helps to create an environment in which end users have better access to more and better-managed data. Such access makes it possible for end users to respond quickly to changes in their environment. 2. Improved data security The more users access the data, the greater the risks of data security breaches. Corporations invest considerable amounts of time, effort, and money to ensure that corporate data are used properly. A DBMS provides a framework for better enforcement of data privacy and security policies. 3. Better data integration Wider access to well-managed data promotes an integrated view of the organization’s operations and a clearer view of the big picture. It becomes much easier to see how actions in one segment of the company affect other segments. 4. Minimized data inconsistency Data inconsistency exists when different versions of the same data appear in different places. For example, data inconsistency exists when a company’s sales department stores a sales representative’s name as “Bill Brown” and the company’s personnel department stores that same person’s name as “William G. Brown,” or when the company’s regional sales office shows the price of a product as $45.95 and its national sales office shows the same product’s price as $43.95. The probability of data inconsistency is greatly reduced in a properly designed database. 5. Improved data access The DBMS makes it possible to produce quick answers to ad hoc queries. From a database perspective, a query is a specific request issued to the DBMS for data manipulation—for example, to read or update the data. Simply put, a query is a question, and an ad hoc query is a spur-of-the-moment question. The DBMS sends back an answer (called the query result set) to the application. For example, end users, when dealing with large amounts of sales data, might want quick answers to questions (ad hoc queries) such as:
- What was the dollar volume of sales by product during the past six months?
- What is the sales bonus figure for each of our salespeople during the past three months?
- How many of our customers have credit balances of 3,000 or more? 6. Improved decision making Better-managed data and improved data access make it possible to generate better-quality information, on which better decisions are based. The quality of the information generated depends on the quality of the underlying data. Data quality is a comprehensive approach to promoting the accuracy, validity, and timeliness of the data. While the DBMS does not guarantee data quality, it provides a framework to facilitate data quality initiatives. 7. Increased end-user productivity The availability of data, combined with the tools that transform data into usable information, empowers end users to make quick, informed decisions that can make the difference between success and failure in the global economy.
DBMS Components: Hardware When we say Hardware, we mean computer, hard disks, I/O channels for data, and any other physical component involved before any data is successfully stored into the memory. When we run Oracle or MySQL on our personal computer, then our computer's Hard Disk, our Keyboard using which we type in all the commands, our computer's RAM, ROM all become a part of the DBMS hardware. DBMS Components: Software This is the main component, as this is the program which controls everything. The DBMS software is more like a wrapper around the physical database, which provides us with an easy-to-use interface to store, access and update data. The DBMS software is capable of understanding the Database Access Language and intrepret it into actual database commands to execute them on the DB. DBMS Components: Data Data is that resource, for which DBMS was designed. The motive behind the creation of DBMS was to store and utilise data. In a typical Database, the user saved Data is present and meta data is stored. Metadata is data about the data. This is information stored by the DBMS to better understand the data stored in it. For example: When I store my Name in a database, the DBMS will store when the name was stored in the database, what is the size of the name, is it stored as related data to some other data, or is it independent, all this information is metadata. DBMS Components: Procedures Procedures refer to general instructions to use a database management system. This includes procedures to setup and install a DBMS, To login and logout of DBMS software, to manage databases, to take backups, generating reports etc. DBMS Components: Database Access Language Database Access Language is a simple language designed to write commands to access, insert, update and delete data stored in any database. A user can write commands in the Database Access Language and submit it to the DBMS for execution, which is then translated and executed by the DBMS. User can create new databases, tables, insert data, fetch stored data, update data and delete the data using the access language.
Codd's Rule for Relational DBMS E.F Codd was a Computer Scientist who invented the Relational model for Database management. Based on relational model, the Relational database was created. Codd proposed some rules popularly known as Codd's 12 rules to test DBMS's concept against his relational model. Codd's rule actualy define what quality a DBMS requires in order to become a Relational Database Management System(RDBMS).
Rule 1: Information rule All information(including metadata) is to be represented as stored data in cells of tables. The rows and columns have to be strictly unordered. Rule 2: Guaranted Access Each unique piece of data(atomic value) should be accesible by : Table Name + Primary Key(Row) + Attribute(column). NOTE: Ability to directly access via POINTER is a violation of this rule. Rule 3: Systematic treatment of NULL Null has several meanings; it can mean missing data, not applicable or no value. It should be handled consistently. Also, Primary key must not be null, ever. Expression on NULL must give null. Rule 4: Active Online Catalog Database dictionary (catalog) is the structure description of the complete Database and it must be stored online. The Catalog must be governed by same rules as rest of the database. The same query language should be used on catalog as used to query database. Rule 5: Powerful and Well-Structured Language One well structured language must be there to provide all manners of access to the data stored in the database. Example: SQL , etc. If the database allows access to the data without the use of this language, then that is a violation. Rule 6: View Updation Rule All the view that are theoretically updatable should be updatable by the system as well. Rule 7: Relational Level Operation There must be Insert, Delete, Update operations at each level of relations. Set operation like Union, Intersection and minus should also be supported. Rule 8: Physical Data Independence The physical storage of data should not matter to the system. If say, some file supporting table is renamed or moved from one disk to another, it should not effect the application. Rule 9: Logical Data Independence If there is change in the logical structure(table structures) of the database the user view of data should not change. Say, if a table is split into two tables, a new view should give result as the join of the two tables. This rule is most difficult to satisfy. Rule 10: Integrity Independence The database should be able to enforce its own integrity rather than using other programs. Key and Check constraints, trigger etc, should be stored in Data Dictionary. This also make RDBMS independent of front-end. Rule 11: Distribution Independence A database should work properly regardless of its distribution across a network. Even if a database is geographically distributed, with data stored in pieces, the end user should get an impression that it is stored at the same place. This lays the foundation of distributed database.
Validity. An FMS should guarantee that at any given moment the stored data reflect the operations performed on them. Protection. Illegal or potentially dangerous operations on the data should be controlled by the FMS. Concurrency. In multiprogramming systems, concurrent access to the data should be allowed with minimal differences. Performance. Compromise data access speed and data transfer rate with functionality. From the point of view of an end user (or application) an FMS typically provides the following functionalities :File creation, modification and deletion. Ownership of files and access control on the basis of ownership permissions. Facilities to structure data within files (predefined record formats, etc). Facilities for maintaining data redundancies against technical failure (back-ups, disk mirroring, etc.). Logical identification and structuring of the data, via file names and hierarchical directory structures. II. Database Management Systems Database Management Systems provide the following advantages and disadvantages:
Advantages Disadvantages Greater flexibility Difficult to learn Good for larger databases Packaged separately from the operating system (i.e. Oracle, Microsoft Access, Lotus/IBM Approach, Borland Paradox, Claris FileMaker Pro) Greater processing power Slower processing speeds Fits the needs of many medium to large- sized organizations
Requires skilled administrators Storage for all relevant data Expensive Provides user views relevant to tasks performed Ensures data integrity by managing transactions (ACID test = atomicity, consistency, isolation, durability) Supports simultaneous access Enforces design criteria in relation to data format and structure Provides backup and recovery controls Advanced security The goals of a Database Management System can be summarized as follows : Data storage, retrieval, and update (while hiding the internal physical implementation details) A user-accessible catalog Transaction support Concurrency control services (multi-user update functionality) Recovery services (damaged database must be returned to a consistent state) Authorization services (security) Support for data communication Integrity services (i.e. constraints) Services to promote data independence Utility services (i.e. importing, monitoring, performance, record deletion, etc.) The components to facilitate the goals of a DBMS may include the following:
Query processor Data Manipulation Language preprocessor Database manager (software components to include authorization control, command processor, integrity checker, query optimizer, transaction manager, scheduler, recovery manager, and buffer manager) Data Definition Language compiler File manager File System v/s DBMS Parameters DBMS File System Meaning DBMS is a collection of data and set of programs to access those data. File system is an abstraction to store, retrieve and update a set of files. Redundancy^ Data^ Repetition of data can be reduced.^ Repetition of data cannot be reduced. Inconsistency^ Data^ Inconsistency of data can be avoided.^ Inconsistency of data cannot be avoided. Inconsistency^ Data^ Inconsistency of data can be avoided.^ Inconsistency of data cannot be avoided. Sharing of Data Data can be shared easily. Data cannot be shared easily. There is difficulty in accessing the data. Data Integrity The data can be accessed from the database through physically as well as logically. Data independence cannot be provided to access the data. Response to Request It gives fast response to information request because the data are integrated into a single database. It gives slow response to information request. Security It restricts unauthorized access. Lack of security. Concurrency Control Data can be accessed by multiple users at same point of time. Problem in concurrency control. Data Recovery It provides back up of data and restores the database to its original state after database failure. It does not provide back up and data recovery. The data may loss if the operation is failed.
Abstraction is one of the main features of database systems. Hiding irrelevant details from user and providing abstract view of data to users, helps in easy and efficient user-database interaction. To understand the view of data, you must have a basic knowledge of data abstraction and instance & schema. Refer these two tutorials to learn them in detail.
- Instance and schema
- Data abstraction DBMS Architecture contains Schemas, Sub-schemas and Instances Schemas: - The overall logical design of the database. Sub-Schemas:- A sub schema is a subset of the schema and inherits the same property that a schema has. The plan (or scheme) for a view is often called sub- schemas. Instances: - The collection of information stored in the database at a particular moment. The Difference between Schema and Instance are given in below table:
COMPARISON^ BASIS FOR^ SCHEMA^ INSTANCE Definition The overall logical design of the database. The collection of information stored in the database at a particular moment. Includes Table names,column names, datatypes, and size of columns, various constraint at logical level.
Actual data or information stored in tables in form of different records. Change changes infrequently. changes frequently. Cause of change insertion of tables or columns and change in datatype,size or constraints on any column. insert,delete or update operation on data stored in database.
Three-level Database Architecture The Architecture of most of commercial dbms are available today is mostly based on this ANSI- SPARC database architecture. THREE-TIER architecture has main three levels:
- Internal Level
- Conceptual Level
- External Level These three levels provide data abstraction which means hide the low level complexities from end users. A database system should be efficient in performance and convenient in use. Using these three levels, it is possible to use complex structures at internal level for efficient operations and to provide simpler convenient interface at external level. 1. Internal level: This is the lowest level of data abstraction. It describes how the data are actually stored on storage devices. It is also known as physical level. It provides internal view of physical storage of data. It deals with complex low level data structures, file structures and access methods in detail. It also deals with Data Compression and Encryption techniques, if used. 2. Conceptual level: This is the next higher level than internal level of data abstraction. It describes what data are stored in the database and what relationships exist among those data. It is also known as logical level. It hides low level complexities of physical storage.
Database administrator and designers work at this level to determine what data to keep in database. Application developers also work on this level.
3. External Level: This is the highest level of data abstraction. It describes only part of the entire database that a end user concern. It is also known as a view level. End users need to access only part of the database rather than entire database. Different user needs different views of database and so, there can be many view level abstractions of the same database.
We have namely two levels of data independence arising from these levels of abstraction:
1. Physical level data independence : It refers to the characteristic of being able to modify the physical schema without any alterations to the conceptual or logical schema, done for optimization purposes, e.g., Conceptual structure of the database would not be affected by any change in storage size of the database system server. Changing from sequential to random access files is one such example. These alterations or modifications to the physical structure may include: Utilizing new storage devices. Modifying data structures used for storage. Altering indexes or using alternative file organization techniques etc. 2. Logical level data independence: It refers characteristic of being able to modify the logical schema without affecting the external schema or application program. The user view of the data would not be affected by any changes to the conceptual view of the data. These changes may include insertion or deletion of attributes, altering table structures entities or relationships to the logical schema etc.
Mappings Process of transforming request and results between three levels it's called Mapping. There are the two types of mappings: Conceptual/Internal Mapping External/Conceptual Mapping 1. Conceptual/Internal Mapping:
The conceptual/internal mapping defines the correspondence between the conceptual view and the store database. It specifies how conceptual record and fields are represented at the internal level. It relates conceptual schema with internal schema. If structure of the store database is changed. If changed is made to the storage structure definition-then the conceptual/internal mapping must be changed accordingly, so that the conceptual schema can remain invariant. There could be one mapping between conceptual and internal levels.
2. External/Conceptual Mapping:
The external/conceptual mapping defines the correspondence between a particular external view and conceptual view. It relates each external schema with conceptual schema.
o It also controls the backup and recovery operations. Data Dictionary: Data Dictionary, which stores metadata about the database, in particular the schema of the database. Names of the tables, names of attributes of each table, length of attributes, and number of rows in each table. Detailed information on physical database design such as storage structure, access paths, files and record sizes. Usage statistics such as frequency of query and transactions. Data dictionary is used to actually control the data integrity, database operation and accuracy. It may be used as a important part of the DBMS Data Files: Which store the database itself. Indices: A database index is a data structure that improves the speed of data retrieval operations on a database table at the cost of additional writes and storage space to maintain the index data structure. Indexes are used to quickly locate data without having to search every row in a database table every time a database table is accessed. Indexes can be created using one or more columns of a database table, providing the basis for both rapid random lookups and efficient access of ordered records.` Compiled DML:- The DML complier converts the high level Queries into low level file access commands known as compiled DML. Database Users: - Database users are the one who really use and take the benefits of database. There will be different types of users depending on their need and way of accessing the database. Sophisticated Users - They are database developers, who write SQL queries to select/insert/delete/update data. They do not use any application or programs to request the database. They directly interact with the database by means of query language like SQL. These users will be scientists, engineers, analysts who thoroughly study SQL and DBMS to apply the concepts in their requirement. In short, we can say this category includes designers and developers of DBMS and SQL. Specialized Users - These are also sophisticated users, but they write special database application programs. They are the developers who develop the complex programs to the requirement. Naive Users - these are the users who use the existing application to interact with the database. For example, online library system, ticket booking systems, ATMs etc which has existing application and users use them to interact with the database to fulfill their requests. Database Administration - Person in the organization who controls the design and the use of the database refers as DBA. o Schema Definition: The DBA definition the logical Schema of the database.A Schema refers to the overall logical structure of the database. According to this schema, database will be developed to store required data for an organization. o Storage Structure and Access Method Definition: The DBA decides how the data is to be represented in the stored database.
o Assisting Application Programmers: The DBA provides assistance to application programmers to develop application programs. o Physical Organization Modification: The DBA modifies the physical organization of the database to reflect the changing needs of the organization or to improve performance. o Approving Data Access: The DBA determines which user needs access to which part of the database. According to this, various types of authorizations are granted to different users. o Monitoring Performance: The DBA monitors performance of the system. The DBA ensures that better performance is maintained by making changes in physical or logical schema if required. o Backup and Recovery: Database should not be lost or damaged. The DBA ensures this periodically backing up the database on magnetic tapes or remote servers. In case of failure, such as virus attack database is recovered from this backup. Query Processor Units: - Executes low level instructions generated by DML compiler. o DDL Interpreter o DML Compiler o Embedded DML Pre-compiler o Query Evaluation Engine Storage Manager Units o Checks the authority of users to access data. o Checks for the satisfaction of the integrity constraints. o Preserves atomicity and controls concurrency. o Manages allocation of space on disk.
o Fetches data from disk storage to memory for being used:
Authorization Manager Integrity Manager Transaction Manager File manager Buffer Manager
Functions of DBMS: o DBMS free the programmers from the need to worry about the organization and location of the data i.e. it shields the users from complex hardware level details. o DBMS can organize process and present data elements from the database. This capability enables decision makers to search and query database contents in order to extract answers that are not available in regular Reports.
Data Administrator (DA): "Person in the organization who controls the data of the database refers data administrator." DA determines what data to be stored in database based on requirement of the organization. DA works on such as requirements gathering, analysis, and design phases. DA does not to be a technical person, any kind of knowledge about database technology can be more beneficiary DA is some senior level person in the organization. in short, DA is a business focused person but should understand about the database technology.
Database Administrator (DBA): "Person in the organization who controls the design and the use of the database refers database administrator." DBA provides necessary technical support for implementing a database. DBA works on such as design, development, testing, and operational phases. DBA is a technical person having knowledge of database technology. DBA does not need to be a business person. in short, DBA is a technically focused person but should understand about the business to administrator the database effectively.
DBA Responsibilities: The following sections examine the responsibilities of the database administrator and how they translate to various Microsoft SQL Server tasks. Installing and Upgrading an SQL Server The DBA is responsible for installing SQL Server or upgrading an existing SQL Server. In the case of upgrading SQL Server, the DBA is responsible for ensuring that if the upgrade is not successful, the SQL Server can be rolled back to an earlier release until the upgrade issues can be resolved. The DBA is also responsible for applying SQL Server service packs. A service pack is not a true upgrade, but an installation of the current version of software with various bug fixes and patches that have been resolved since the product's release. Monitoring the Database Server's Health and Tuning Accordingly Monitoring the health of the database server means making sure that the following is done: The server is running with optimal performance. The error log or event log is monitored for database errors. Databases have routine maintenance performed on them, and the overall system has periodic maintenance performed by the system administrator. Using Storage Properly SQL Server 2000 enables you to automatically grow the size of your databases and transaction logs, or you can choose to select a fixed size for the database and transaction log. Either way, maintaining the proper use of storage means monitoring space requirements and adding new storage space (disk drives) when required. Performing Backup and Recovery Duties Backup and recovery are the DBA's most critical tasks; they include the following aspects: Establishing standards and schedules for database backups Developing recovery procedures for each database Making sure that the backup schedules meet the recovery requirements Managing Database Users and Security With SQL Server 2000, the DBA works tightly with the Windows NT administrator to add user NT logins to the database. In non-NT domains, the DBA adds user logins. The DBA is
also responsible for assigning users to databases and determining the proper security level for each user. Within each database, the DBA is responsible for assigning permissions to the various database objects such as tables, views, and stored procedures. Working with Developers It is important for the DBA to work closely with development teams to assist in overall database design, such as creating normalized databases, helping developers tune queries, assigning proper indexes, and aiding developers in the creation of triggers and stored procedures. In the SQL Server 2000 environment, a good DBA will show the developers how to use and take advantage of the SQL Server Index Tuning Wizard and the SQL Server profiler. Establishing and Enforcing Standards The DBA should establish naming conventions and standards for the SQL Server and databases and make sure that everyone sticks to them. Transferring Data The DBA is responsible for importing and exporting data to and from the SQL Server. In the current trend to downsize and combine client/server systems with mainframe systems and Web technologies to create Enterprise systems, importing data from the mainframe to SQL Server is a common occurrence that is about to become more common with the SQL Server 2000 Data Transformation Services. Replicating Data SQL Server version 2000 has many different replication capabilities such as Merge replication (2-way disconnected replication) and queued replication. Managing and setting up replication topologies is a big undertaking for a DBA because of the complexities involved with properly setting up and maintaining replication. Data Warehousing SQL Server 2000 has substantial data warehousing capabilities that require the DBA to learn an additional product (the Microsoft OLAP Server) and architecture. Data warehousing provides new and interesting challenges to the DBA and in some companies a new career as a warehouse specialist. Scheduling Events The database administrator is responsible for setting up and scheduling various events using Windows NT and SQL Server to aid in performing many tasks such as backups and replication. Providing 24-Hour Access The database server must stay up, and the databases must always be protected and online. Be prepared to perform some maintenance and upgrades after hours. Also be prepared to carry that dreaded beeper. If the database server should go down, be ready to get the server up and running. After all, that's your job. Learning Constantly To be a good DBA, you must continue to study and practice your mission-critical procedures, such as testing your backups by recovering to a test database. In this business, technology changes very fast, so you must continue learning about SQL Server, available client/servers, and database design tools. It is a never-ending process.
DBA should posses the following skills (1) A good knowledge of the operating system (2) A good knowledge of physical database design
ER model represents the all these entities, attributes and their relationship in the form of picture to make the developer understand the system better. It also list attributes of each objects. In the below diagram, Entities or real world objects are represented in a rectangular box. Their attributes are represented in ovals. Primary keys of entities are underlined. All the entities are mapped using diamonds. This is one of the methods of representing ER model. There are many different forms of representation. More details of this model are described in ER data model article.
ER model is a graphical representation of real world objects with their attributes and relationship. It makes the system easily understandable. This model is considered as a top down approach of designing a requirement.
Advantages
It makes the requirement simple and easily understandable by representing simple diagrams. One can covert ER diagrams into record based data model easily. Easy to understand ER diagrams
Disadvantages
No standard notations are available for ER diagram. There is great flexibility in the notation. It’s all depends upon the designer, how he draws it. It is meant for high level designs. We cannot simplify for low level design like coding.
Object Oriented Data Models
This data model is another method of representing real world objects. It considers each object in the world as objects and isolates it from each other. It groups its related functionalities together and allows inheriting its functionality to other related sub-groups. Example - Let us consider an Employee database to understand this model better. In this database we have different types of employees – Engineer, Accountant, Manager, Clark. But all these employees belong to Person group. Person can have different attributes like name, address, age and phone. What do we do if we want to get a person’s address and phone number? We write two separate procedure sp_getAddress and sp_getPhone. What about all the employees above? They too have all the attributes what a person has. In addition, they have their EMPLOYEE_ID, EMPLOYEE_TYPE and DEPARTMENT_ID attributes to identify them in the organization and their department. We have to retrieve their department details, and hence we sp_getDeptDetails procedure. Currently, say we need to have only these attributes and functionality.
Since all employees inherit the attributes and functionalities of Person, we can re-use those features in Employee. But do we do that? We group the features of person together into class. Hence a class has all the attributes and functionalities. For example, we would create a person class and it will have name, address, age and phone as its attribute, and sp_getAddress and sp_getPhone as procedures in it. The values for these attributes at any instance of time are object. i.e. ; {John, Troy, 25, 2453545 : sp_getAddress (John), sp_getPhone (John)} forms on person object. {Mathew, Fraser Town, 28, 5645677: sp_getAddress (Mathew), sp_getPhone (Mathew} forms another person object. Now, we will create another class called Employee which will inherit all the functionalities of Person class. In addition it will have attributes EMPLOYEE_ID, EMPLOYEE_TYPE and DEPARTMENT_ID, and sp_getDeptDetails procedure. Different objects of Employee class are Engineer, Accountant, Manager and Clerk.
Here we can observe that the features of Person are available only if other class is inherited from it. It would be a black box to any other classes. This feature of this model is called encapsulation. It binds the features in one class and hides it from other classes. It is only visible to its objects and any inherited classes. Advantages Because of its inheritance property, we can re-use the attributes and functionalities. It reduces the cost of maintaining the same data multiple times. Also, these informations are encapsulated and, there is no fear being misused by other objects. If we need any new feature we can easily add new class inherited from parent class and adds new features. Hence it reduces the overhead and maintenance costs. Because of the above feature, it becomes more flexible in the case of any changes. Codes are re-used because of inheritance. Since each class binds its attributes and its functionality, it is same as representing the real world object. We can see each object as a real entity. Hence it is more understandable. Disadvantages It is not widely developed and complete to use it in the database systems. Hence it is not accepted by the users.
