DATABASE MANAGEMENT SYSTEMS/CHAPTER-1

Written by:-  Ramu dayinaboyina   For lecturer reference

Main points only for quick reference taken from c.j.date

A database is simply a computer based record keeping system

It is a system whose overall purpose is to record and maintain the information

This information is concerned with the data that is significant to an organization

The major components that involves in the database system are

v  Data

v  Hardware

v  Software

v  Users

Data: - the data stored in the system is the forms of databases

A database is a repository of stored data

In general it is both     “integrated”   and         “shared”

The integrated data means that the database may be thought of as a unification of several distinct data files

Shared data means the indidual pieces of data in the database that may be shared among several users

Hardware

It consists of the secondary storage devices such as disks, tapes…etc on which the database is stored

The hardware also includes the devices that are associated above devices

Software it is the layer between the data and the hardware usually called as the database management system (DBMS)

All the requests from the user for access to the database are handled by the DBMS

It provides the interface between data and hardware

Users

The users are broadly classified as

Application programmers

End users

Database administrators (DBA)

The application programmers are responsible for writing programs that use the database, typically in any language (for example COBOL)

The end users are who access the data from the terminal

The end user may use the query language provided by the system

Or

Application programs written by the programmers

The database administrator   Is a person Or A group of persons who has the centralized control over the data?

Operational data

The data in the database referred to as the operational data this does not include the input output and other kinds of data

The data maintained by an enterprise for its operations is called operational data

Now the database can be defined as the collection of stored operational data used by the application of some particular enterprise

Entity:-An entity is a distinguishable object that is to be represented in the database

Relationship: - The associations or linking the basic entities together is called relationship

The same entity may be associated in any number of relation ships

Advantages of database

A database is used to have the centralized control over the operational data in an enterprise

 There is a responsible person having centralized control as the data base who is called database administrator (DBA)

The advantages of maintaining the database with a control on the operational data are

1. Redundancy can be reduced

Redundancy Means   maintaining the data of an item in two areas this leads to the complications and wastage of space

This can be reduced by maintaining a database

2. Inconsistency can be avoided

Inconsistency means a data having 2 entities are not agreeing

A database in an inconsistent state is capable of supplying incorrect and conflicting information this inconsistency can be avoided to the maximum extinct by maintaining the database

3 data can be shared

If the database is manipulated for the enterprise then it can be shared for different applications, new applications can also be developed operating on the same stored data

4. Security restrictions can be applied

Providing security for the data is the most important aspect

If the database is maintained then we have a control on the data so we can ensure that the database is accessed only through proper channel we can also check the unauthorized access to the sensitive data

5. Integrity can be maintained

Integrity means bringing similarities among the entities with the control on the database the entities can be integrated so that the inconsistency and redundancy of data can be avoided

6. Standreds can be enforced to the data stored in the database must maintained in the same standard storing data in different or various formats.

This is particularly describes in interchanging the data between different systems

7. Conflicting requirements can be balanced

The database provides the overall control on the data so the requirements of the data management in the enterprise can be clearly understood and it can be balanced

DATA INDEPENDENCE

“Data dependence means”

               The way in which the data is organized in the secondary storage devices and the way in which it is accessed both are dictated by the requirements of the application

              The knowledge of the data organatision and access technique used in the logic of the application

            So it is impossible to change storage structure and access method without modifying the application

The immunity of an application change in storage structure and access methods are called the data independence

Such application don’t depend on any particular storage structure or devices and access methods strategies

    The data independence is extremely describe for the applications as

1. Different applications will need different views of same data

For example if the data in a file can be stored in binaries and the data in other file stored in decimals then we can’t interchange the data between them

The DBMS should provide a facility for conversion of the data stored in different formats

2. The DBA must have the freedom to change the storage structures or access strategy for any requirement without modifying the existing application

If the data is independence then it is possible to change the storage structure, type, access methods, in different applications

Logical independence

Physical independence

DATABASE ARCHITECTURE

                                 The database architecture describes the general database concepts and explains the structure of individual system

The architecture is divided into three general levels

1.       External level

2.       Conceptual level

3.       Internal level

The internal level is the one closet to the physical storage, and it is concerned with the way in which the data is actually stored

The external level is the one closest to the users those concerned with the the way in which the data is viewed by individual users.

The conceptual level is a level of indirection that defines the community user’s view

The 3 levels of an employee record is

External level (COBOL)

01 EMP-REC

      05   EMP-NO    PIC    9(5)

      05   NAME       PIC    A(20)

      05   SALARY   PIC    9(4)

Conceptual level

Employee Record

Employee Number   Numeric (5)

Employee Name                   Char (20)

Salary                                 Name (4)

Internal Level

EMP-REC      length=29 bytes

EMP-NO    Bytes (5)            Offcet-0

NAME       Bytes (20)          Offcet-5

SALARY   Bytes (4)            Offcet-25

The components of database architecture can be explained in detail as follows

HL host language DSL data sub language

The users are either application programmers, end-users even DBAs

Each user has a language (such as COBOL) to access the database

The end users has a query language or a readymade program

Either of these 2 is called DSL (Data Sublanguages)

The DSL is a subset of total language used with the database

The DSL is a part embedded in a Host Language

The DSL is the combination of 2 languages

I.e. DataDefinationLanguage (DDL) and DataManipulationLanguage (DML)

The DDL provides the definition or description of the database object The DDL consists of declarations

The DDL is written outside of the program

the DML consists of executable instructions

The DML supports the manipulations or processing of such objects

The DML is called with statements such as CALL

In some cases, both DDL and DML are included in the same language

The individual user is generally interested only in some portion of the database

He is not concerned with how it is stored but how it is seen  His view is called external view

So an external view is the content of the database as it is seen by some particular user.

The external view consists of multiple occurrences of multiple types of external records.

An external record is not necessarily same as stored record

Each external view is defined by means of an external schema

The external schema consists of definitions of each of the various types of external records in the external view.

The external schema is written using the DDL of DSL

The conceptual view is a representation of the entire information content of the database

It is the abstract of the data stored physically on the devices

The conceptual view consists of multiple occurrences of multiple types of conceptual records.

A conceptual record is not necessacarly the same as either an external record or a stored record

The conceptual schema defines the conceptual view

The conceptual schema contains definitions of various types of conceptual records

The conceptual schema is written using the conceptual DDL

The third level of the architecture is the internal level

The internal view is very low level representation of the entire database

It consists of multiple occurrences of multiple types of internal records.

An internal record is the physically stored records.

The internal schema defines the internal view

This internal schema defines the type, indexing method and physical stored sequence of the internal record

In the database architecture there are 2 levels of mapping

One mapping is between external and conceptual levels and the other mapping is Between conceptual and internal levels

The conceptual /internal mapping defines the correspondence between the conceptual view and the stored database

It specifies how conceptual records map into the corresponding the stored records

The external /conceptual mapping defines the correspondence between a particular external view and the conceptual view

The DBMS is the software that handles all access to the database

The DBMS includes the activities as

1)      A user issues an access request using  a DML

2)      The DBMS interprets these requests

3)      The DBMS inspects the schemas and mappings

4)      The DBMS performs the necessary operations on the stored database

DATABASE ADMINISTRATOR

            The another compartment in the architecture of database system is DBA

The DBA is a person or a group of persons responsible for overall control of the database system

The DBA responsibilities includes the following

1)      Deciding the information content of the database

It is the DBA’s job to decide exactly what information is to be held in the database the DBA must define the content of the database by writing the conceptual schema

2)      Deciding the storage structure and accessing strategy

The DBA must also decide how the data is to be represented in the database and

Must specify the representation by writing the storage structure definitions using a DDL

The associated mapping between the storage structure definition and the conceptual schema must also be specified

3)      Interacting with users

The DBA must interact with the users to ensure that the data …required is available

And to write the necessary external schemas using different external DDL’s

4)      Defining authorizing checks and validation procedures

The authorization checks and validation procedures are considered as logical extensions of conceptual schema

The conceptual DDL should include the facilities for specifying such checks and procedures

5)      Defining a storage for backup and recovery

Damage to any portion of database may be caused due to human error or hardware failure etc   so the DBA must implement the appropriate strategy for backup the database into tapes and has to implement the procedures for reloading (recovering )the relevant portion of the database from the tapes

6)      Monitoring performances and responding to change in requirements

The DBA is responsible for organizing the system to get the best performance

He requires a number of utility programs such as data dictionary

DISTRIBUTED DATABASE

A distributed database is a database that is not stored in its entity at a single physical storage location

But it is spread across a network of computers that are geographically dispersed and connected via communication links

For example

A central bank has its braches at different locations

A branch office may has a record of a customer

This record is stored any of that Brach office

But this can be distributed to other braches as wells to the central office.

The advantage of a distributed network is that a centralized system is provided to the users

The disadvantage is that data transfer depends on communications links

So it is risky and needs a lot of technical expertise

STORAGE STRUCUTRE

The database is stored as a secondary storage device with certain structure and certain access method

The DBMS connects the internal records into external and vice versa

The components responsible for this convention is called the AccessMehtod

The accessmehtod consists of a set of program routine which conceals all the device dependent details fromDBMS and provides a “STORED RECORD INTERFACE”.

The interface permits the DBMS to view the storage structure as a collection of stored files consisting of stored records each consisting of stored fields

The access method is responsible for assembling a unique” stored record address (SRA)”. This value is used to identify a record in the database uniquely.

-----End of the Chapter -------