Chapter2 Database Environment

Chapter2 Database Environment

2.1The Three-Level ANSI-SPARC Architecture
the ANSI-SPARC (the american national standards institute (ANSI)Standards Planning and Requirement Committee(SPARC))architecture recognized the
need for a three-level approach with a system catalog.

2.1.1 External Level:

this level describe that part of the database that is relvant to each user.

2.1.2 Conceptual Level:

describes what data is stored in the database the the relationship among the data.

2.1.3 Internal Level:

describe how the data is stored in the database.

2.1.4Schema,Mappings,and Instances

The overall description of the database is called the database system.

At the highest level,we have multiple external schema that correspond to different view of the data.

At the conceptual level,we have the conceptual

schema,which describe all the entites,attributes,and relationship together with integrity constraints.

At the lowest level of abstraction ,we have the internal schema,which is a complete description of the internal model,containing the definitions of stored record,the methods of representation,the data fields ,and the indexes and storage structures used.

The DBMS is responsible for mapping between these three type of schema.

...The conceptual schema is related to the internal schema through a conceputal/internal schema.This mapping enables the DBMS to find the

actual record or combination of records in physical storage that consitute a logical record in the conceptual schema...

Finally,each external schema is related to the conceptual schema by the external/conceptual mapping.

It is important to distinguish between the description of the database and database itself....The schema is sometimes called the intension of the database,an instance is called an extension of the database.

2.1.5 Data Independence

A major objective of the three-level architecture is to provide 

data independence.

Logical data independence:

Physical data independence:

2.2Database Languages
2.2.1The Data Definition Language
2.2.2The Data Manipulation Language
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2.3 Data Models and Conceptual Modeling
Data model:An integrated collection of concepts for describing and manipulating data ,relationships between data,and constraints on the data in
an organization.
1. Structural part,consisting of a set of rules according to which databases can be constructed.
2. Manipulative part .dedining the types of operation that are allowed on the data operations that are used for updating or retrieving data from
the database and for changing the structure of the database.
3. A set of integrity constraints,which ensures that the data is accurate.To reflect the ANSI-SPARC architecture introduced in section 2.1,we can
identity three related data models:

1. the external data model object-based

2. the conceptual data model record-based

3. the internal data model physical

2.3.1Object-Based Data Model

use concepts such as entities , attributes and relationship.

2.3.2Record-Base Data Models
• Relational data model based on the concept of mathematical relations.In the relational model,data and relationship are represented as tables.
• Network data model data is represented as collections of records, and relationship are represented by set.

• Hierachical data model is a restricted type of network model...however , the hierarchical model allows the node to have only one parent.

2.3.3Physical Data Models
2.3.4Conceptual Modeling
2.4Functions of a DBMS

1. data storage
2. a user-accessible catalog
3. transaction support
4. concurrency control services
5. authorization services
6. support for data communication
7. integrity services
8. services to promote data independence
9. utility services