A database is a collection of logically related data designed to meet the information needs of an organization. It includes entities, attributes, and relationships. A database management system (DBMS) enables users to define, create, maintain, and control access to the database. It allows application programs to interact with the database using queries. Key components of a DBMS environment include hardware, software, data, procedures, and users who have different roles. DBMSs provide advantages like data sharing and integrity but also disadvantages like complexity, costs, and potential single points of failure.

1. Database
> Shared collection of logically related data (and a description of this
data), designed to meet the information needs of an organization.
> Systemcatalog (metadata) provides description of data to enable
program–data independence.
> Logically related data comprises entities, attributes, and relationships
of an organization’s information.
Database Management System(DBMS)
> A softwaresystemthatenables users to define, create, maintain, and
control access to the database.
> (Database) application program: a computer program that interacts
with databaseby issuing an appropriaterequest (SQL statement) to the
DBMS.
Database Approach
Data definitionlanguage (DDL).
>Permits specification of data types, structures and any data constraints.
>All specifications are stored in the database.
Data manipulationlanguage (DML).
>General enquiry facility (query language) of the data.
Components of DBMS Environment
Hardware
Can range froma PC to a network of computers.
Software
DBMS, operating system, network software(if necessary) and also the
application programs.
Data
Used by the organization and a description of this data called the
schema
Procedures
Instructions and rules that should be applied to the design and useof
the databaseand DBMS.
People

2. Roles in the Database Environment
>Data Administrator (DA)
>Database Administrator (DBA)
>Database Designers (Logicaland Physical)
>Application Programmers
>End Users (naive and sophisticated)
Advantages of DBMSs
>Control of data redundancy
>Data consistency
>More information fromthe same amount of data
>Sharing of data
>Improved data integrity
>Improved security
>Enforcement of standards
>Economy of scale
>Balance conflicting requirements
>Improved data accessibility and responsiveness
>Increased productivity
>Improved maintenancethrough data independence
>Increased concurrency
>Improved backup and recovery services
Disadvantages of DBMSs
>Complexity
>Size
>Cost of DBMS
>Additional hardwarecosts
>Cost of conversion
>Performance
>Higher impact of a failure
ANSI-SPARC Architecture
External Level
*Users’ view of the database.
*Describes that part of databasethat is relevant to a particular user.
Conceptual Level

3. *Community view of the database.
*Describes whatdata is stored in databaseand relationships among the
data.
Internal Level
*Physicalrepresentation of the databaseon the computer.
*Describes how the data is stored in the database.
Relational Keys
Superkey
* An attribute, or set of attributes, that uniquely identifies a tuple within
a relation.
Candidate Key
* Superkey (K) such that no proper subsetis a superkey within the
relation.
* In each tuple of R, values of K uniquely identify that tuple
(uniqueness).
* No proper subsetof K has the uniqueness property (irreducibility).
Primary Key
*Candidate key selected to identify tuples uniquely within relation.
Alternate Keys
*Candidate keys that are not selected to be primary key.
ForeignKey
*Attribute, or set of attributes, within one relation that matches
candidate key of some(possibly same) relation.
Integrity Constraints
Null
> Represents value for an attribute that is currently unknown or not
applicable for tuple.
> Deals with incomplete or exceptional data.
> Represents the absence of a value and is not the same as zero or
spaces, which are values.
Entity Integrity
>In a base relation, no attribute of a primary key can be null.
Referential Integrity
>If foreign key exists in a relation, either foreign key value mustmatch a
candidate key value of sometuple in its home relation or foreign key
value must be wholly null.
General Constraints
>Additional rules specified by users or databaseadministrators that
define or constrain someaspect of the enterprise.

4. Entity Type
Strong Entity Type
Entity type that is not existence-dependent on some other entity type.
Weak Entity Type
Entity type that is existence-dependent on some other entity type.