by Dinesh Thakur

dBase is a specific software product used for creating and manipulating relational databases. The term can also refer to the dBase database programming language that first appeared in the dBase product, but which is now available in many other database forms.

 
by Dinesh Thakur

Data recovery is the art of restoring lost or damaged files. This damage can occur when your computer crashes, a virus infects, you accidentally reformat a disk that contains precious data, or you experience some other catastrophe of considerable dimension. And, at some point in your life, you're going to delete a file you really didn't mean to (believe me). The next time tragedy strikes, try running one of the many data recovery applications (powerful software written specifically for data recovery purposes) to see if it can correct the situation. Often these little jewels work magic and save your day-and your files.



 
by Dinesh Thakur

In a computer database, the database engine is the software that does the real work of sorting the information, finding specific data that you request, and so on. The term used to refer to a separate piece of software that ran on a central computer (in this case, it is more or less synonymous with the term "back-end"). Widely used database engines include Oracle, DB2, and Sybase. Separate front-end software running on your own computer lets you tell the database engine what to do (how to sort the data, what data to find), and displays the results of your commands.



 
by Dinesh Thakur

A database document is just a collection of information stored in computerized form. The simplest way to understand a database is to think of it like a set of 3 x 5 cards. Since the information is on your computer, though, a dick of the mouse or the stroke of a key can alphabetize those "cards," or find just the names of the people on the cards who live in a certain town, or tell you who owes how much money, and so on.



 
by Dinesh Thakur

Attributes means characteristics. For instance, in a database or a spreadsheet you can apply attributes to each field or cell to customize your document. As a general attribute, you can choose whether it is to be a text field or a numeric field or perhaps a computed field, whose value the application calculates for you.

 
by Dinesh Thakur

Relation schema:

A set of attributes is called a relation schema (or relation scheme). A relation schema is also known as table schema (or table scheme). A relation schema can be thought of as the basic information describing a table or relation. It is the logical definition of a table. Relation schema defines what the name of the table is. This includes a set of column names, the data types associated with each column.



 
by Dinesh Thakur

A functional dependency is an association between two attributes of the same relational database table. One of the attributes is called the determinant and the other attribute is called the determined. For each value of the determinant there is associated one and only one value of the determined.

 
by Dinesh Thakur

The E-R model can result problems due to limitations in the way the entities are related in the relational databases. These problems are called connection traps. These problems often occur due to a misinterpretation of the meaning of certain relationships.

 
by Dinesh Thakur

The entity set which does not have sufficient attributes to form a primary key is called as Weak entity set. An entity set that has a primary key is called as Strong entity set. Consider an entity set Payment which has three attributes: payment_number, payment_date and payment_amount. Although each payment entity is distinct but payment for different loans may share the same payment number. Thus, this entity set does not have a primary key and it is an entity set. Each weak set must be a part of one-to-many relationship set.

 
by Dinesh Thakur

There is no standard for representing data objects in ER diagrams. Each modeling methodology uses its own notation.

All notational styles represent entities as rectangular boxes and relationships as lines connecting boxes. Each style uses a special set of symbols to represent the cardinality of connection. The symbols used for the basic ER constructs are:

 
by Dinesh Thakur

In Entity-Relationship model a database is modeled as a collection of entities and relationship among entities. The ER model views the real world as a construct of entities and association between entities.

 
by Dinesh Thakur

There are two techniques used for the purpose of data base designing from the system requirements. These are:

• Top down Approach known as Entity-Relationship Modeling

• Bottom Up approach known as Normalization.

 
by Dinesh Thakur

Relational model stores data in the form of tables. This concept purposed by Dr. E.F. Codd, a researcher of IBM in the year 1960s. The relational model consists of three major components:

 
by Dinesh Thakur

The Network model replaces the hierarchical tree with a graph thus allowing more general connections among the nodes. The main difference of the network model from the hierarchical model, is its ability to handle many to many (N:N) relations. In other words, it allows a record to have more than one parent. Suppose an employee works for two departments. The strict hierarchical arrangement is not possible here and the tree becomes a more generalized graph - a network. The network model was evolved to specifically handle non-hierarchical relationships. As shown below data can belong to more than one parent. Note that there are lateral connections as well as top-down connections. A network structure thus allows 1:1 (one: one), l: M (one: many), M: M (many: many) relationships among entities.

 
by Dinesh Thakur

A model is a representation of reality, 'real world' objects and events, associations. It is an abstraction that concentrates on the essential, inherent aspects an organization and ignores the accidental properties. A data model represents the organization itself. It should provide the basic concepts and notations that will allow database designers and end users unambiguously and accurately to communicate their understanding of the organizational data.

 
by Dinesh Thakur

The DBMS can be classified according to the number of users and the database sitelocations. These are:

On the basis of the number of users:

• Single-user DBMS

• Multi-user DBMS



 
by Dinesh Thakur

A DBMS performs several important functions that guarantee integrity and consistency of data in the database. Most of these functions are transparent to end-users. There are the following important functions and services provided by a DBMS:



 
by Dinesh Thakur

A typical structure of a DBMS with its components and relationships between them is show. The DBMS software is partitioned into several modules. Each module or component is assigned a specific operation to perform. Some of the functions of the DBMS are supported by operating systems (OS) to provide basic services and DBMS is built on top of it. The physical data and system catalog are stored on a physical disk. Access to the disk is controlled primarily by as, which schedules disk input/output. Therefore, while designing a DBMS its interface with the as must be taken into account.



 
by Dinesh Thakur

A DBMS must provide appropriate languages and interfaces for each category of users to express database queries and updates. Database Languages are used to create and maintain database on computer. There are large numbers of database languages like Oracle, MySQL, MS Access, dBase, FoxPro etc. SQL statements commonly used in Oracle and MS Access can be categorized as data definition language (DDL), data control language (DCL) and data manipulation language (DML).



 
by Dinesh Thakur

A metadata (also called the data dictionary) is the data about the data. It is the self describing nature of the database that provides program-data independence. It is also called as the System Catalog. It holds the following information about each data element in the databases, it normally includes:



 
by Dinesh Thakur

Functions of Data Base Administrator

Data Base Administrator (DBA) is a person or group in charge for implementing DBMS in an organization. Database Administrator's job requires a high degree of technical expertise and the ability to understand and interpret management requirements ata senior level. In practice the DBA may consist of team of people rather than just one person



 
by Dinesh Thakur

Any access to the stored data is done by the data manager. A user's request for data is-received by the data manager, which detern1ines the physical record required. The decision as 10 which physical record is needed may require some preliminary consultation of the database and/or the data dictionary prior to the access of the actual data itself.

 
by Dinesh Thakur

A major objective for three-level architecture is to provide data independence, which means that upper levels are unaffected by changes in lower levels.

There are two kinds of data independence:

 
by Dinesh Thakur

Database changes over time when information is inserted or 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.



 
by Dinesh Thakur

DBMS A database management system is the software system that allows users to define, create and maintain a database and provides controlled access to the data.

A Database Management System (DBMS) is basically a collection of programs that enables users to store, modify, and extract information from a database as per the requirements. DBMS is an intermediate layer between programs and the data. Programs access the DBMS, which then accesses the data. There are different types of DBMS ranging from small systems that run on personal computers to huge systems that run on mainframes. The following are main examples of database applications:



 
by Dinesh Thakur

In order to remove all limitations of the File Based Approach, a new approach was required that must be more effective known as Database approach

The Database is a shared collection of logically related data, designed to meet the information needs of an organization. A database is a computer based record keeping system whose over all purpose is to record and maintains information. The database is a single, large repository of data, which can be used simultaneously by many departments and users. Instead of disconnected files with redundant data, all data items are integrated with a minimum amount of duplication.



 
by Dinesh Thakur

File processing systems was an early attempt to computerize the manual filing system that we are all familiar with. A file system is a method for storing and organizing computer files and the data they contain to make it easy to find and access them. File systems may use a storage device such as a hard disk or CD-ROM and involve maintaining the physical location of the files.



 
by Dinesh Thakur

Hierarchical Database Model

Hierarchical Database model is one of the oldest database models, dating from late 1950s. One of the first hierarchical databases Information Management System (IMS) was developed jointly by North American Rockwell Company and IBM. This model is like a structure of a tree with the records forming the nodes and fields forming the branches of the tree.

 
by Dinesh Thakur

The Database Users fall into several categories:


Application Programmers are computer professionals interacting with the system through DML calls embedded in a program written in a host language (e.g. C, PL/1, Pascal): These programs are called Application Programs. The DML Precompiled converts DML calls (prefaced by a special character like $, #, etc.) to normal procedure calls in a host language.



 
by Dinesh Thakur

Database System Structure are partitioned into modules for different functions. Some functions (e.g. file systems) may be provided by the operating system. Components include:



 
by Dinesh Thakur

There are four basic components of Database Management System:

 

(i) Data: Raw facts which we want to feed in the computer.

(ii) Hardware: On which the data to be processed.

(iii) Software: The interface between the hardware and user, by which the data   will change into the information.

(iv) User: There are so many types of users some of them are application programmer, end case users and DBA.



 
by Dinesh Thakur

A Database Administrator, Database Analyst or Database Developer is the person responsible for managing the information within an organization. As most companies continue to experience inevitable growth of their databases, these positions are probably the most solid within the IT industry.



 
by Dinesh Thakur

SQL statements are divided into two major categories: data definition language (DDL) and data manipulation language (DML).

Data Definition Language (DDL) statements are used to define the database structure or schema. Some examples:

 
by Dinesh Thakur

When multiple transactions are trying to access the same sharable resource, there could arise many problems if the access control is not done properly. There are some important mechanisms to which access control can be maintained. Earlier we talked about theoretical concepts like serializability, but the practical concept of this can be implemented by using Locks and Timestamps.

 
by Dinesh Thakur

What is a database server? It is similar to data warehouse where the website store or maintain their data and information. A Database Server is a computer in a LAN that is dedicated to database storage and retrieval. The database server holds the Database Management System (DBMS) and the databases. Upon requests from the client machines, it searches the database for selected records and passes them back over the network.



 
by Dinesh Thakur

What is a Database View, A view can join information from several tables together, or we can say that Views are useful for Hiding unwanted information for example adding the ename field to the Order information. Database View is a subset of the database sorted and displayed in a particular way. A database view displays one or more database records on the same page. A view can display some or all of the database fields.



 
by Dinesh Thakur

The environment of database is said to be instance. A database instance or an ‘instance’ is made up of the background processes needed by the database software. These processes usually include a process monitor, session monitor, lock monitor, etc.  They will vary from database vendor to database vendor.

 
by Dinesh Thakur

A database object in a relational database is a data structure used to either store or reference data. The most common object that most people interact with is the table. Other objects are indexes, stored procedures, sequences, views and many more.

 
by Dinesh Thakur

A database instance controls 0 or more databases. A database contains 0 or more database application schemas. A database application schema is the set of database objects that apply to a specific application. These objects are relational in nature, and are related to each other, within a database to serve a specific functionality. 

 
by Dinesh Thakur

The related information when placed is an organized form makes a database. In simple words, an organized collection of related information is known as database. The organization of data/information is necessary because unorganized information has no meaning. There are so many examples of organized information, more precisely and the most common are, the dictionary, the telephone directory, student record register, your own address book and many more. In each of these the data is stored in some particular order i.e. in an organized form.



 
by Dinesh Thakur

An early proposal for a standard terminology and general architecture for database systems was produced in 1971 by the DBTG (Data Base Task Group) appointed by the Conference on Data Systems and Languages (CODASYL, 1971). The DBTG recognized the need for a two level approach with a system view called the schema and user views called subschema. The American National Standards Institute (ANSI) Standards Planning and Requirements Committee (SPARC) produced a similar terminology mid architecture in 1975 (ANSI 1975). ANSI-SPARC recognized the need for a three level approach with a system catalog.

 
by Dinesh Thakur

Normalization is the process of removing redundant data from your tables in order to improve storage efficiency, data integrity and scalability. This improvement is balanced against an increase in complexity and potential performance losses from the joining of the normalized tables at query-time. There are two goals of the normalization process: eliminating redundant data (for example, storing the same data in more than one table) and ensuring data dependencies make sense (only storing related data in a table). Both of these are worthy goals as they reduce the amount of space a database consumes and ensure that data is logically stored.

 
by Dinesh Thakur

When a company asks you to make them a working, functional Database Management System (DBMS) which they can work with, there are certain steps to follow. Let us summarize them here:



 
by Dinesh Thakur

DBMS stand for Database Management System, which consist n number of tables there is no relationship between another table. RDMBS stand for Relational Database Management System, which having the relationship with other tables. The Relationship between tables in DBMS is Physical and the relationship in RDBMS is Logical.