Download database management system

14. Databases
14.1
DATABASE
MANAGEMENT
SYSTEM
DBMS components
 A database is a collection of data that is logically,
but not necessarily physically, coherent.
 A database management system (DBMS) defines,
creates, and maintains a database and allows
controlled access to users.
14.2
ARCHITECTURE
Database architecture
Database architecture
 DBMS users can be humans or application programs.
 A DBMS has three levels: internal, conceptual, and
external.
 The internal level of a DBMS interacts directly with the
hardware and is concerned with low-level access
methods and byte transfer to and from the storage device.
 The conceptual level of a DBMS defines the logical view
of the data as well as the data model and schema
diagrams.
 The external level of a DBMS interacts directly with the
user.
14.3
DATABASE
MODELS
Database models
A database model defines the logical
design of data
The model also describes the relationships
between different parts of data.
Three models:
Hierarchical model
Network model
Relational model
Hierarchical model
Data are organized as an upside down tree
Each entity has only one parent but can
have several children.
Network model
The entities are organized in a graph,
where some entities can be accessed
through several path.
14.4
RELATIONAL
MODEL
Relational model
 Data are organized in two-dimensional tables
called relations.
 The relational database management system
(RDBMS) is the only database model in wide
use today. The hierarchical and network models
are obsolete.
 Each column in a relation is called an attribute.
The number of attributes in a relation is its
degree.
 Each row in a relation is called a tuple. The
number of rows in a relation is its cardinality.
Relational model
Relation
14.5
OPERATIONS
ON
RELATIONS
Insert operation
None operations can be performed on
relations.
An operation that operates on one relation
is a unary operator. Unary operators
include the insert, delete, update, select,
and project operations.
Delete operation
Update operation
Select operation
Project operation
Join operation
An operation that operates on two relation
is a binary operator. binary operators
include the join, union, intersection, and
difference operations.
Union operation
Intersection operation
Difference operation
14.6
STRUCTURED
QUERY
LANGUAGE
SQL
 The Structured Query Language (SQL) is the
language standardized by ANSI and ISO for use
on relational databases.
 For example: Fig 14.10
select
*
整個tuple都抓出來
from
COURSES
where
Unit = 5
SQL- cont’s
For example:
select
Name
from
where
CourseCOURSES
Unit = 5