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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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182-7a
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