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get any particular point in time, there would be one particular set of records exists in each of the objects, satisfying all the conditions of a database is called? from screen.
6 Database ManagementChapter 6
Database Management6.1 Hierarchy of Data [Figure 6.1][Slide 6-4]
Data are the principal resources of an organization. Data stored in computer systems form a hierarchy extending from a single bit to a database, the major record-keeping entity of a firm. Each higher rung of this hierarchy is organized from the components below it.
Data are logically organized into:
1. Bits (characters)
2. Fields 3. Records 4. Files 5. Databases
(Character) - a bit is the smallest unit of data representation (value of a bit may be a 0 or 1). Eight bits make a byte which can represent a character or a special symbol in a character code.
- a field consists of a grouping of characters. A data field represents an attribute (a characteristic or quality) of some entity (object, person, place, or event).
- a record represents a collection of attributes that describe a real-world entity. A record consists of fields, with each field describing an attribute of the entity.
- a group of related records. Files are frequently classified by the application for which they are primarily used (employee file). A in a file is the field (or fields) whose value identifies a record among others in a data file.
- is an integrated collection of logically related records or files. A database consolidates records previously stored in separate files into a common pool of data records that provides data for many applications. The data is managed by systems software called database management systems (DBMS). The data stored in a database is independent of the application programs using it and of the types of secondary storage devices on which it is stored.
6.2 File Environment and its Limitations
There are three principal methods of organizing files, of which only two provide the direct access necessary in on-line systems.File Organization [Figure 6.2 & 6.3]
Data files are organized so as to facilitate access to records and to ensure their efficient storage. A tradeoff between these two requirements generally exists: if rapid access is required, more storage is required to make it possible.
to a record for reading it is the essential operation on data. There are two types of access:
1. - is performed when records are accessed in the order they are stored. Sequential access is the main access mode only in batch systems, where files are used and updated at regular intervals.
2. - on-line processing requires direct access, whereby a record can be accessed without accessing the records between it and the beginning of the file. The primary key serves to identify the needed record.
There are three methods of file organization: [Table 6.1]
1. Sequential organization
2. Indexed-sequential organization
3. Direct organizationSequential Organization
In sequential organization records are physically stored in a specified order according to a key field in each record.
Advantages of sequential access:
1. It is fast and efficient when dealing with large volumes of data that need to be processed periodically (batch system).
Disadvantages of sequential access:
1. Requires that all new transactions be sorted into the proper sequence for sequential access processing.
2. Locating, storing, modifying, deleting, or adding records in the file requires rearranging the file.
3. This method is too slow to handle applications requiring immediate updating or responses.Indexed-Sequential Organization
In the indexed-sequential files method, records are physically stored in sequential order on a magnetic disk or other direct access storage device based on the key field of each record. Each file contains an index that references one or more key fields of each data record to its storage location address.Direct Organization
Direct file organization provides the fastest direct access to records. When using direct access methods, records do not have to be arranged in any particular sequence on storage media. Characteristics of the direct access method include:
1. Computers must keep track of the storage location of each record using a variety of direct organization methods so that data can be retrieved when needed.
2. New transactions' data do not have to be sorted.
3. Processing that requires immediate responses or updating is easily performed.6.3 Database Environment [Figure 6.6][Slide 6-5]
A database is an organized collection of interrelated data that serves a number of applications in an enterprise. The database stores not only the values of the attributes of various entities but also the relationships between these entities. A database is managed by a database management system (DBMS), a systems software that provides assistance in managing databases shared by many users.
1. Helps organize data for effective access by a variety of users with different access needs and for efficient storage.
2. It makes it possible to create, access, maintain, and control databases.
3. Through a DBMS, data can be integrated and presented on demand.
Advantages of a database management approach:
1. Avoiding uncontrolled data redundancy and preventing inconsistency
2. Program-data independence
3. Flexible access to shared data
[MCQs] Database Management Systems
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[MCQs] Database Management Systems
Introduction Database Concepts
Entity–Relationship Data Model
Relational Model and relational Algebra
Structured Query Language (SQL)
Transactions Management and Concurrency
Introduction Database Concepts
Module 011. A relational database consists of a collection of
a) Tables b) Fields c) Records d) Keys Answer: a
Explanation: Fields are the column of the relation or tables. Records are each row in a relation. Keys are the constraints in a relation.2. A ________ in a table represents a relationship among a set of values.
a) Column b) Key c) Row d) Entry Answer: c
Explanation: Column has only one set of values. Keys are constraints and row is one whole set of attributes. Entry is just a piece of data.3. The term _______ is used to refer to a row.
a) Attribute b) Tuple c) Field d) Instance Answer: b
Explanation: Tuple is one entry of the relation with several attributes which are fields.4. The term attribute refers to a ___________ of a table.
a) Record b) Column c) Tuple d) Key Answer: b
Explanation: Attribute is a specific domain in the relation which has entries of all tuples.5. For each attribute of a relation, there is a set of permitted values, called the ________ of that attribute.
a) Domain b) Relation c) Set d) Schema Answer: a
Explanation: The values of the attribute should be present in the domain. Domain is a set of values permitted.Crack Job Placement Aptitude in First Attempt
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Click Here!6. Database __________ which is the logical design of the database, and the database _______ which is a snapshot of the data in the database at a given instant in time.
a) Instance, Schema b) Relation, Schema c) Relation, Domain d) Schema, Instance Answer: d
Explanation: Instance is an instance of time and schema is a representation.7. Course(course_id,sec_id,semester)
Here the course_id,sec_id and semester are __________ and course is a _________
a) Relations, Attribute
b) Attributes, Relation
c) Tuple, Relation
d) Tuple, Attributes
Explanation: The relation course has a set of attributes course_id,sec_id,semester .8. Department (dept name, building, budget) and Employee (employee_id, name, dept name, salary)Here the dept_name attribute appears in both the relations. Here using common attributes in relation schema is one way of relating ___________ relations.
a) Attributes of common
b) Tuple of common
c) Tuple of distinct
d) Attributes of distinct
Explanation: Here the relations are connected by the common attributes.9. A domain is atomic if elements of the domain are considered to be ____________ units.
a) Different b) Indivisbile c) Constant d) Divisible Answer: b Explanation: None.10. The tuples of the relations can be of ________ order.
a) Any b) Same c) Sorted d) Constant Answer: a
Explanation: The values only count. The order of the tuples does not matter.Crack Job Placement Aptitude in First Attempt
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Click Here!11. Using which language can a user request information from a database?
a) Query b) Relational c) Structural d) Compiler Answer: a
Explanation: Query language is a method through which the database entries can be accessed.12. Student(ID, name, dept name, tot_cred)In this query which attributes form the primary key?
a) Name b) Dept c) Tot_cred d) ID Answer: d
Explanation: The attributes name, dept and tot_cred can have same values unlike ID.13. Which one of the following is a procedural language?
a) Domain relational calculus
b) Tuple relational calculus
c) Relational algebra
d) Query language Answer: c
Explanation: Domain and Tuple relational calculus are non-procedural language. Query language is a method through which database entries can be accessed.14. The_____ operation allows the combining of two relations by merging pairs of tuples, one from each relation, into a single tuple.
a) Select b) Join c) Union d) Intersection Answer: b
Explanation: Join finds the common tuple in the relations and combines it.15. The result which operation contains all pairs of tuples from the two relations, regardless of whether their attribute values match.
b) Cartesian product
c) Intersection d) Set difference Answer: b
Explanation: Cartesian product is the multiplication of all the values in the attributes.Python Programming for Complete Beginners
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Click Here!16. The _______operation performs a set union of two “similarly structured” tables
Chapter 1 Before the Advent of Database Systems – Database Design – 2nd Edition
CHAPTER 1 BEFORE THE ADVENT OF DATABASE SYSTEMS
The way in which computers manage data has come a long way over the last few decades. Today’s users take for granted the many benefits found in a database system. However, it wasn’t that long ago that computers relied on a much less elegant and costly approach to data management called the file-based system.
One way to keep information on a computer is to store it in permanent files. A company system has a number of application programs; each of them is designed to manipulate data files. These application programs have been written at the request of the users in the organization. New applications are added to the system as the need arises. The system just described is called the file-based system.
Consider a traditional banking system that uses the file-based system to manage the organization’s data shown in Figure 1.1. As we can see, there are different departments in the bank. Each has its own applications that manage and manipulate different data files. For banking systems, the programs may be used to debit or credit an account, find the balance of an account, add a new mortgage loan and generate monthly statements.
Figure 1.1. Example of a file-based system used by banks to manage data.
Disadvantages of the file-based approach
Using the file-based system to keep organizational information has a number of disadvantages. Listed below are five examples.
Often, within an organization, files and applications are created by different programmers from various departments over long periods of time. This can lead to data redundancy, a situation that occurs in a database when a field needs to be updated in more than one table. This practice can lead to several problems such as:
Inconsistency in data format
The same information being kept in several different places (files)
Data inconsistency, a situation where various copies of the same data are conflicting, wastes storage space and duplicates effort
Data isolation is a property that determines when and how changes made by one operation become visible to other concurrent users and systems. This issue occurs in a concurrency situation. This is a problem because:
It is difficult for new applications to retrieve the appropriate data, which might be stored in various files.
Problems with data integrity is another disadvantage of using a file-based system. It refers to the maintenance and assurance that the data in a database are correct and consistent. Factors to consider when addressing this issue are:
Data values must satisfy certain consistency constraints that are specified in the application programs.
It is difficult to make changes to the application programs in order to enforce new constraints.
Security can be a problem with a file-based approach because:
There are constraints regarding accessing privileges.
Application requirements are added to the system in an ad-hoc manner so it is difficult to enforce constraints.
Concurrency is the ability of the database to allow multiple users access to the same record without adversely affecting transaction processing. A file-based system must manage, or prevent, concurrency by the application programs. Typically, in a file-based system, when an application opens a file, that file is locked. This means that no one else has access to the file at the same time.
In database systems, concurrency is managed thus allowing multiple users access to the same record. This is an important difference between database and file-based systems.
The difficulties that arise from using the file-based system have prompted the development of a new approach in managing large amounts of organizational information called the database approach.
Databases and database technology play an important role in most areas where computers are used, including business, education and medicine. To understand the fundamentals of database systems, we will start by introducing some basic concepts in this area.
Role of databases in business
Everybody uses a database in some way, even if it is just to store information about their friends and family. That data might be written down or stored in a computer by using a word-processing program or it could be saved in a spreadsheet. However, the best way to store data is by using database management software. This is a powerful software tool that allows you to store, manipulate and retrieve data in a variety of different ways.
Most companies keep track of customer information by storing it in a database. This data may include customers, employees, products, orders or anything else that assists the business with its operations.
The meaning of data
Data are factual information such as measurements or statistics about objects and concepts. We use data for discussions or as part of a calculation. Data can be a person, a place, an event, an action or any one of a number of things. A single fact is an element of data, or a data element.
If data are information and information is what we are in the business of working with, you can start to see where you might be storing it. Data can be stored in: