A database is a model of some aspect of the reality of an organization.
It is conventional to call this reality a universe of discourse (UoD), or sometimes a domain of discourse. A UoD will be made up of classes and relationships between classes. The classes in a UoD will be defined in terms of their properties or attributes.
A database can be considered as an organized collection of data which is meant to represent some UoD.
Data are facts. A datum, a unit of data, is one symbol or a collection of symbols that is used to represent something.
Facts by themselves are meaningless. To prove useful they must be interpreted.
Information is interpreted data. Information is data placed within a meaningful context.
Information is data with an assigned semantics or meaning.
A database can be considered as a collection of facts or positive assertions about a UoD, such as relational database design is a module and John Davies takes relational database design.
Usually negative facts, such as what modules are not taken by a student, are not stored. Hence, databases constitute ‘closed worlds’ in which only what is explicitly represented is regarded as being true.
A database is said to be in a given state at a given time. A state denotes the entire collection of facts that are true at a given instant in time. A database system can therefore be considered as a factbase which changes through time.
Data in a database is described as being persistent. By persistent we mean that the data is held for some duration. The duration may not actually be very long.
The term persistence is used to distinguish more permanent data from data which is more transient in nature.
Hence, product data, account data, patient data and student data would all normally be regarded as examples of persistent data. In contrast, data input at a personal computer, held for manipulation within a program, or printed out on a report, would not be regarded as persistent, as once it has been used it is no longer required.
A database is made up of two parts: an intentional part and an extensional part. The intension of a database is a set of definitions which describe the structure or organization of a given database. The extension of a database is the total set of data in the database. The intension of a database is also referred to as its schema. The activity of developing a schema for a database system is referred to as database design.

Data sharing
Data stored in a database is not usually held solely for the use of one person. A database is normally expected to be accessible by more than one person, perhaps at the same time. Hence a students’ database might be accessible by members of not only academic but also administrative staff.
Data integration
One major responsibility of database usage is to ensure that the data is integrated. This implies that a database should be a collection of data, which, at least ideally, has no redundant data.
In the past, for instance separate files of student information might have been maintained by different academic and administrative departments of a university with many Fields in common. The aim of a database system would be to store one logical item of data in one place only. Hence, one student record would be accessible to a range of information systems.
Data integrity
Another responsibility arising as a consequence of shared data is that a database should display integrity. In other words, that the database should accurately reflect the universe of discourse that it is attempting to model.
Data security
One of the major ways of ensuring the integrity of a database is by restricting access. In other words, securing the database. The main way that this is done in contemporary database systems is by defining in some detail a set of authorized users of the whole, or more usually parts of the database. For instance, a secure system would be one where the finance department has access to information used for the collection of student fees but is prohibited from changing the fee levels of given students.
Data abstraction
A database can be viewed as a model of reality. The information stored in a database is usually an attempt to represent the properties of some objects in the real world.
Data independence
One immediate consequence of abstraction is the idea of buffering data from the processes that use such data. The idea is to achieve a situation where data organization is transparent to the users or application programs which feed off data. If for instance, a change is made to some part of the underlying database no application programs using affected data should need to be changed. Also, if a change is made to some part of an application system then this should not affect the structure of the underlying data used by the application.
A database management system (DBMS) is an organized set of facilities for accessing and maintaining one or more databases. A DBMS is a shell which surrounds a database or series of databases and through which all interactions take place with the database.
The interactions catered for by most existing DBMS fall into three main groups
1. Data maintenance – Adding new data structures to the database, removing data structures from the database, modifying the structure of existing data, inserting new data into existing data structures, updating data in existing data structures, deleting data from existing data structures.
2. Data Retrieval – Querying existing data by end -users and extracting data for use by application programs.
3. Data control -data in the database and monitoring the performance of databases. Data Maintenance Information Retrieval Database Administration Database Data Management Subsystem

DBMS Components
Data: Data stored in a database include numerical data which may be integers (whole numbers only) or floating point numbers (decimal), and non-numerical data such as characters (alphabetic and numeric characters), date or logical (true or false). More advanced systems may include more complicated data entities such as pictures and images as data types.
Standard operations: Standard operations are provided by most DBMS. These operations provide the user basic capabilities for data manipulation. Examples of these standard operations are sorting, deleting and selecting records.
Data definition language (DDL): DDL is the language used to describe the contents of the database. It is used to describe, for example, attribute names (field names), data types, location in the database, etc.
Data manipulation and query language: Normally a query language is supported by a DBMS to form commands for input, edit, analysis, output, reformatting, etc. Some degree of standardization has been achieved with SQL (Structured Query Language).
Programming tools: Besides commands and queries, the database should be accessible directly from application programs through function calls (subroutine calls) in conventional programming languages.
File structures: Every DBMS has its own internal structures used to organize the data although some common data models used by most DBMS.
Data Manager – The Data Manager is the central software component of the DBMS also knows
Data management subsystem

A Database Control System. The Main Functions Of Data Manager Are: –

• Convert operations in user’s Queries coming from the application programs or combination of DML Compiler and Query optimizer which is known as Query Processor from user’s logical view to physical file system.

• Controls DBMS information access that is stored on disk.

• It also controls handling buffers in main memory.

• It also enforces constraints to maintain consistency and integrity of the data.

• It also synchronizes the simultaneous operations performed by the concurrent users.

• It also controls the backup and recovery operations. Data Dictionary – Data Dictionary is a repository of description of data in the database. It contains information about

• Data – names of the tables, names of attributes of each table, length of attributes, and number of rows in each table. • Relationships between database transactions and data items referenced by them which is useful in determining which transactions are affected when certain data definitions are changed.

• Constraints on data i.e. range of values permitted.

• Detailed information on physical database design such as storage structure, access paths, files and record sizes.

• Access Authorization – is the Description of database users their responsibilities and their access rights.

• 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. Importance of Data Dictionary – Data Dictionary is necessary in the databases due to following reasons:

• It improves the control of DBA over the information system and user’s understanding of use of the system. • It helps in documenting the database design process by storing documentation of the result of every design phase and design decisions. • It helps in searching the views on the database definitions of those views.

• It provides great assistance in producing a report of which data elements (i.e. data values) are used in all the programs.

• It promotes data independence i.e. by addition or modifications of structures in the database application program are not affected.



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