DBMS Data Models

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Schemas versus Instances

Database Schema: The description of adatabase which is specified during databasedesign and is not expected to change frequently.Includes descriptions of the database structureand the constraints that should hold on thedatabase.

Schema Diagram: A diagrammatic display of(some aspects of) a database schema.

Schema Construct: A component of theschema or an object within the schema, e.g.,STUDENT, COURSE.


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Physical schema: database design at thephysical level

Logical schema: database design at the

logical level

Database Instance: The actual datastored in a database at aparticularmoment in time. Also called databasestate (oroccurrence or snapshot).


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The goal of the three-schema architecture is to separate

the user applications and the physical database.

The schemas can be divided into three levels:

internal levelhas an internal schema, which describes

the physical storage structure of the database Conceptual levelhas a conceptual schema hides the

details of physical storage structures and concentrates

on describing entities, data types, relationships, user

operations and constraints.

External or view levelincludes a number of external

schemas or user views.


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Data Independence

Data independence can be defined as the capacity tochange the schema at one level of a database system

without having to change the schema at the next

higher level.

Two types of data independences are :

Logical data independence: is the capacity to change

the conceptual schema without having to change

external schemas or application programs. Physical data independence: is the capacity to change

the internal schema without having to change the

conceptual schema.


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Data Model: A set of concepts to describe the structure of a

database, and certain constraints that the database should obey.

data types

relationships

Provide data abstraction

Data Model Operations: Operations for specifying database

retrievals and updates by referring to the concepts of the data

model.

generic operation: insert, delete, modify, retrieve

user-defined operations

Data Models, Schemas, and Instances


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Categories of Data Models:

- Conceptual (high-level, semantic) data models: Provide concepts that

are close to the way many users perceive data.(Also called entity-based or object-based data models.)

entity attribute relationship

- Physical (low-level, internal) data models: Provide concepts thatdescribe details of how data is stored in the computer.

record formats record ordering access paths

- Implementation (record-oriented) data models: Provide concepts that

fall between the above two, balancing user views with some computer

storage details.

relational network hierarchical


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Conceptual Models

High level

Example: Entity-relationship diagrams

Uses concepts such as Entity: Real-world thing that is described in

the database

Attribute: Property of an entity

Relationship: Association between entities


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Logical Models

More specific than conceptual models

Follow rules of a particular method of

implementing a databaseMost common: Relational model

Others Hierarchical

Network

Object-oriented


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Physical Data Models

Low-level

Describe how the database is actually

stored on the system

DBMS takes care of much of this detail


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Three Schema Architecture

Logicalindependence

Physical

independence


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Entity-Relationship Model

Example of schema in the entity-

relationship model


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122-4a

Schema diagram forUNIVERSITY database

schema construct

Known data:

name of record types, data items


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Figure 1.2

UNIVERSITY Database


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define

empty state

initial state

load

state

update

updatevalid state

satisfy database schema


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2.2 DBMS Architecture and Data Independence

Proposed to support DBMS characteristics of:

- Insulation of programs and data/program and operations

(program-data and program-operation independence)

- Support of multiple views of the data.

- Use of catalog (database description)

Defines DBMS schema at three levels: (see 2-9)

- Internal schema at the internal level to describe data storage structures and access

paths. Typically uses aphysicaldata model.

- Conceptual schema at the conceptual level to describe the structure and constraints

for the whole database. Uses a conceptualor an implementation data model.- External schema at the external level to describe the various user views. Usually

uses the same data model as the conceptual level or high-level data model.

Mappings among schema levels are also needed. Programs refer to an external schema,

and are mapped by the DBMS to the internal schema for execution

2.2.1 Three-Schema Architecture


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Figure 2.2 The Three-schema architecture 2-6


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2.2.2 Data Independence

Logical Data Independence: The capacity to change the conceptual schema withouthaving to change the external schemas and their application programs.

Physical Data Independence: The capacity to change the internal schema without

having to change the conceptual schema.

When a schema at a lower level is changed, only the mappings between this

schema and higher-lever schemas need to be changed in a DBMS that fully supports

data independence. The higher-level schemas themselves are unchanged. Hence, the

application programs need not be changed since they refer to the external schemas.

By adding or removing a record type or data

item to

expand the database (2-11)

reduce the database

Reorganize physical files to improve performance

e.g. List all sections offered in Fall 1998

Disadvantages of two levels of mappings:

Overhead during compilation or execution of a query or program


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UNIVERSITY Conceptual Schema

STUDENT (Name, Student Number, Class, Major)

COURSE (Course Name, Course Number, Credit, Dept)

PREREQUISITE (Course Number, Prerequisite Number)

SECTION (Section Id, Course Number, Semester, Year, Instructor)GRADE_REPORT(Student Number, Section Id , Grade)

UNIVERSITY External Schema

TRANSCRIPT(Student Name, Course Number, Grade, Semester, Year, Section Id)

derived from STUDENT, SECTION, GRADE_REPORTPREREQUISITES(Course Name, Course Number, Prerequisites)

derived from PREREQUISITE, COURSE

Change GRADE-REPORT Schema Construct

GRADE_REPORT (Student Number, Student Name, Section Id, Course Number, Grade)

Change Mapping (& View Definition)

TRANSCRIPT derived from SECTION, GRADE_REPORT


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2.3 Database Languages and Interfaces

Data Definition Language (DDL): Used by the DBA and database designers to

specify the conceptual schema of a database. In many DBMSs, the DDL is also

used to define internal and external schemas (views). In some DBMSs, separate

storage definition language (SDL) and view definition language (VDL) are

used to define internal and external schemas.

Data Manipulation Language (DML): Used to specify database retrievals and

updates (insertion, deletion, modifications)

- DML commands (data sublanguage) can be embedded in a general-purpose

programming language (host language).

- Alternatively, stand-alone DML commands can be applied directly (query

language).

provide appropriate languages and interfaces for each category of users.

2.3.1 DBMS Languages

DDL Compiler


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Types of DML

-Procedural DML:

Also called record-at-a-time (record-oriented) or low-level DML

Must be embedded in a programming language.

Searches for and retrieves individual database records and uses looping

and other constructs of the host programming language to retrieve multiple

records.

-Declarative or non-procedural DML: Also called set-at-a-time (set-oriented) or high-level DML.

Can be used as a stand-alone query language or can be embedded in a

programming language.

Searches for and retrieves information from multiple related database

records in a single command.

- host language: general-purpose language

- data sublanguage: DML

- C++


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2.3.2 DBMS Interfaces

- Stand-alone query language interfaces. (casual end user)

- Programmer interfaces for embedding DML in programming

languages: (programmer)

-Pre-compiler Approach

-Procedure (Subroutine) Call Approach

- User-friendly interfaces:

-Menu-based Interfaces for Browsing.-Forms-based Interfaces.

-Graphical User Interfaces.

-Natural language Interfaces

-Combination of the above

-Interfaces for Parametic Users (using function keys)

- Interfaces for the DBA:

-Creating accounts, granting authorizations

-Setting system parameters-Changing schemas or access path


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2.4.1 DBMS Component Modules

2.4 The Database System Environment

Figure 2.3


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2.4.2 Database System Utilities

To perform certain functions such as:

-Loadingdata stored in files into a database. Conversion tool

-Backing up the database periodically on storage.-File reorganizingdatabase file structures.

-Report generation utilities.

-Performance monitoringutilities.

- Other functions, such assorting, user monitoring,data compression, etc.


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Data dictionary utility:

- Used to store schema descriptions and other information such as design

decisions, application program descriptions, user information, usage

standards, etc. (comment)

-Active data dictionary is accessed by DBMS software and users/DBA.

-Passive data dictionary is accessed by users/DBA only.

Communications Facilities- Allow users at locations remote from the database system site to access

the database.

DB (DBMS)/DC (Data Communication System)

2.4.3 Tools, Application Environments, and

Communications Facilities


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2.5 Classification of Database Management Systems

Based on the data model used:

Data models

-Traditional: Relational, Network (see 2-19), Hierarchical

- Emerging: Object-oriented, Semantic, Entity- Relationship, other.

Other classifications:

Number of users : Single-user (typically used with personal computers) vs.

multi-user (most DBMSs)

Number of sites:

Centralized (uses a single computer) vs. distributed (uses multiple computers).

Homogeneous vs. Heterogeneous

Cost of DBMS software. $10,000~100,000

$100~3,000Types of access paths used. (inverted file structures, )

Purpose general purpose

special purpose

e.g. airline reservations, telephone directory, on-line transaction

processing system


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Figure 2.4 A Network Schema

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DBMS Data Models