Muhammad Azhar
Muhammad Azhar answered
The relational model is based on the concept of a relation. A relation is a table, which consists of a set of named columns and a set of unnamed rows. The tables are the most important part of the database and hold information about objects. A relation is represented as a two dimensional table, in … Read more
Muhammad Azhar
Muhammad Azhar answered
The halogens form a homologous series but fluorine differs from the other halogens in many respects which is due to: (I)      Small size of F atom and of F~ ion. (ii)      High first ionization energy and electronegativity. (iii)    Low dissociation energy of F2 molecule as compared to C12 and Br2. … Read more
Muhammad Azhar
Muhammad Azhar answered
Although it is not a metal but in most of the modern versions of Periodic Table, hydrogen is placed at the top of the group IA. This is because of the fact that some of the properties of hydrogen resemble with those of alkali metals. Like alkali metals hydrogen atom has one electron in its … Read more
Muhammad Azhar
Muhammad Azhar answered
A computer network is a set of two or more computers connected together in order to share information and other resources. The computers in a network are connected with one another through cables, satellite or telephone lines. Different types of computers in a network are as follows: Network Components: Different components of a network are … Read more
Muhammad Azhar
Muhammad Azhar answered
Data communication software is a type of software that is used to correspond with other computers. The process of data traveling in the form of software is usually called protocol. This protocol is a set of rules for interchanging information between computers. These protocols define: • How the communication channel is established • How information … Read more
Muhammad Azhar
Muhammad Azhar answered
1. Delivery The system must deliver data to the correct destination. Data must be received by the intended device or user. 2. Accuracy The system must deliver data accurately. Data that have been altered in transmission and left uncorrected are unusable. 3. Timeliness The system should transfer data within time. Data becomes useless if it … Read more
Muhammad Azhar
Muhammad Azhar answered
The way in which data is transmitted from one place to another is called data transmission mode. There are three types of data transmission modes: • Simplex mode • Half duplex mode • Full duplex mode 1. Simplex Mode In simplex mode, data can flow only in one direction. It cannot be moved in both … Read more
Muhammad Azhar
Muhammad Azhar answered
Often one user writes a program to be used by another user. In these cases, misuse can occur and lead to unexpected behavior. The two most common methods by which such behavior may occur are Trojan horses and trap doors. Trojan Horse: One of the most serious threats to computer security is Trojan horse attack. … Read more
Muhammad Azhar
Muhammad Azhar answered
Threat monitoring is a management technique that can improve a security system. This system can easily check any suspicious activity to violate security. A good example of threat monitoring is when a user is attempting to log in. The' system may count the number of incorrect passwords given when trying to login. After a few … Read more
Muhammad Azhar
Muhammad Azhar answered
BOYCE - CODD NORMAL FORM (BCNF): The Third Normal Form (3NF) provides no problems for relations that have single candidate key (primary key). When a relation has more than one candidate key, and candidate keys are composite then overlapping anomalies may occur. Therefore, an improved and best form of Third Normal Form (3NF) is named … Read more
Muhammad Azhar
Muhammad Azhar answered
In 1981, Fagin proposed a simple normal form kno\^n as Domain-Key Normal Form (DK/NF). This normal form involves the concepts of domain- key, and constraints. This normal form of relation cannot have insertion, deletion, and modification anomalies. So the Domain-Key Normal Form is defined as:A relation is in DK/NF, if every constraint on the relation … Read more
Muhammad Azhar
Muhammad Azhar answered
External view level: The external view level is closest to the users. It is concerned with the way the data is viewed by individual users. You can say that external level is the individual user level. A user can either be an application programmer or an end-user, but DBA is an important special case. The … Read more
Muhammad Azhar
Muhammad Azhar answered
Insertion anomaly indicates that we cannot insert a fact about one entity until we have an additional fact about another entity. Suppose we want to store the information that the cost of car is Rs. 14,00,000, but we cannot enter this data into the relation until the data about the car is entered into the … Read more
Muhammad Azhar
Muhammad Azhar answered
Dr. Codd is the founder of relational database model. He published a two-part article that lists 12 rules (or principles). These rules are known as Codd's 12 rules. Dr. Codd's rules provide very useful measure (yardstick) for evaluating a relational database system. These rules are used to determine whether a DBMS is relational and to … Read more
Muhammad Azhar
Muhammad Azhar answered
Relational Algebra is a theoretical language. The relational algebra is not a user friendly language. Relational algebra defines relational operators that are used to perform various operations on relations. Relations can be manipulated using these operators to achieve desired results. Relational algebra is not directly used in commercial database processing, because no commercial DBMS product … Read more
Muhammad Azhar
Muhammad Azhar answered
Relational data model is the most commonly used data model to design the database. In 1970, Dr. Codd gave the concept of relational model, while he was working as a researcher at IBM. The relational model is based on mathematical theory, and therefore, has a solid theoretical foundation. This model represents a database as collection … Read more
Muhammad Azhar
Muhammad Azhar answered
States of Matter: Matter exists in four states, i.e., solids, liquids, gases and plasma.  General Properties of Gases:  1.  Gases do not have a definite volume and occupy all available space. The volume of the gas is volume of the container.  2.  They do not have a definite shape and take the shape of the … Read more
Muhammad Azhar
Muhammad Azhar answered
The momentary forces of attraction that exist between instantaneous dipole and induced dipole are called instantaneous dipole-induced dipole forces or London Dispersion forces. Explanation: These forces are created among the non polar molecules like He. Ne, Ar, Fb, Ch, CH4, which have no dipoles at normal conditions. London forces are responsible for the liquefaction of … Read more
Muhammad Azhar
Muhammad Azhar answered
The three-level architecture is also known as three-schema architecture. The purpose of the three-level architecture is to separate the user applications and the physical database. The reasons of this separation are: (I)        Different users need different views of the same data. (ii)      Users should not have to deal directly with … Read more
Muhammad Azhar
Muhammad Azhar answered
Direct access is based on a disk model of a file. For direct access, the file is viewed as a numbered sequence of block or records. A direct-access file permits random blocks to be read or written. After block 18 has been read, block 57 could be next and then block 3. There are no … Read more