instructors: churee techawut introduction to database system chapter 1 cs (204)321 database system i
TRANSCRIPT
Instructors: Churee Techawut
Introduction to Database System
Chapter 1
CS (204)321 Database System I
< PART 2 >
Outlines
1) Basic definitions 2) Database system environment 3) Examples of database 4) Typical DBMS functionality 5) Major characteristics of database approach 6) Different types of database users 7) Additional characteristics of database approach 8) When not to use a DBMS 9) Components of a database system
10) Database system concepts and architecture
Database System Concepts and Architecture
1) Data models
2) Schemas VS. instances
3) Three-schema architecture
4) Data Independence
5) DBMS language
6) DBMS Interface
7) Database system environment
8) Database system utilities
9) Database architectures
10) Classification of DBMS
Data Models
“Operations for specifying database retrievals and updates by referring to the concepts of the data model.”
Operations on the data model may include basic operations and user-defined operations.
(e.g. A user-defined operation is COMPUTE_GPA which can be applied to a STUDENT object.)
Data model operations
Data model
“A set of concepts that can be used to describe the structure of a database (data types and relationships) and certain constraints that the database should obey.”
Data Models
Categories of data models
1) Conceptual (high-level, semantic) data models:
Provide concepts that are close to the way many users perceive data.
2) Physical (low-level, internal) data models:
Provide concepts that describe the details of how data is stored in the computer.
3) Implementation (representational) data models:
Provide concepts that fall between above two, balancing user views with some computer storage details.
Schemas VS. Instances
In any data model it is important to distinguish between the description of the database and the database itself.
“The description of database. It includes description of database structure and the constraints that should hold on the database.”
The database schema is specified during database design and is not expected to change frequently.
e.g. Name: string
StudentNumber: string
Class: integer
Major: string
Database schema (or meta-data)
Schemas VS. Instances
Schema diagram
“A diagrammatic display of a database schema – structure of each record type (not the actual instances of a record).”
Name StudentNumber Class Major
STUDENT
CourseName CourseNumber CreditHours Department
COURSE
CourseNumber PrerequisiteNumber
PREREQUITSITE
SectionIdentifier CourseNumber Semester Year Instructor
SECTION
StudentNumber SectionIdentifier Grade
GRADE_REPORT
Schemas VS. Instances
Schema construct
“An object within the schema.”
e.g. STUDENT, COURSE.
Database instances
“The actual data stored in a database at a particular moment in time. Also called database state or occurrence.”
Many database instances can be constructed to correspond to a particular database schema.
Schemas VS. Instances
DBMScatalog
empty state
Database
initial state
Data firstly loaded
Database
Update operation
Database
DBMS ensures
valid state
Define a new DB
Specify DB schema
Schemas VS. Instances
Distinction
The database schema does not frequently change, but the database state changes every time the database is updated.
Schema is also called intension, whereas state is called extension.
Three-Schema Architecture
The goal of the three-schema architecture is to separate the user applications and the physical database.
The three-schema architecture was proposed to support DBMS characteristics of :
Program-data independence.
Supporting multiple views of the data.
Three-Schema Architecture
Schema can be defined at the following three level.
1) Internal schema at the internal level
Describes physical storage structures and access paths.
Typically uses a physical data model.
2) Conceptual schema at the conceptual level
Describes the structure (such as entities, data type, relationship) and constraints for the whole database.
Uses a conceptual or implementation data model.
3) External schemas at the external level
Describes the various user views.
Usually uses the same data model as the conceptual level.
Three-Schema Architecture
Source: Elmasri R. & Navathe S.B. (1994) Fundamentals of database systems.
External level
Conceptual level
Internal level
EXTERNAL VIEW1
EXTERNAL VIEWn
END USERS
CONCEPTUAL SCHEMA
INTERNAL SCHEMA
STORED DATABASE
Three-Schema Architecture
Programs refer to an external schema, and are mapped by the DBMS to the internal schema for execution.
Mappings among schema levels are needed to transform requests and data.
Notice that the three schemas are only descriptions of data; the only data that actually exists is at the physical level.
If the request is a database retrieval, the data extracted from the stored database must be reformatted to match the user’s external view.
Data Independence
Two types of data independence:
1) Logical data independenceThe capacity to change the conceptual schema without having to change the external schemas and their application programs.
2) Physical data independenceThe capacity to change the internal schema without having to change the conceptual (or external) schema.
Data Independence
When a schema at a lower level is changed, only the mappings between this schema and higher-level schemas need to be changed in a DBMS that fully supports data independence.
The higher-level schemas themselves are unchanged. Therefore, the application programs need not be changed since they refer to the external schemas.
DBMS Language
Data Definition Language (DDL) is used by the DBA and by database designers to define the conceptual schema for the database and any mapping between the two.
Storage Definition Language (SDL) is used to specify the internal schema.
Once the design of a database is completed and a DBMS is chosen to implement the database:
View Definition Language (VDL) are used to specify external schema - user views and their mappings to the conceptual schema.
DBMS Language
Once the database schemas are compiled and the database is populated with data:
Data Manipulation Language (DML) are used to specify database retrievals and updates.
DML commands (data sublanguage) can be embedded in a general-purpose programming language (host language), such as COBOL, C or an Assembly Language.
In object-oriented systems, the host and data sublanguages typically form one integrated language such as C++.
Alternatively, a high-level DML used in stand-alone interactive manner is called a query language.
DBMS Language
Types of DML
1) Procedural DML (record-at-a-time or low-level DML)
Must be embedded in a programming language.
Typically retrieve individual records from the database, and use looping and other constructs of the host programming language to retrieve multiple records.
Specify how to retrieve data.
e.g. COBOL, C, etc.
DBMS Language
2) Declarative or Non-procedural DML (set-at-a-time or high-level DML)
Use as a stand-alone query language or embedded in a programming language.
Typically retrieves information from multiple related database records in a single command.
Specify what data to retrieve than how to retrieve.
Also called declarative languages.
e.g. SQL
DBMS Interface
Stand-alone query language interfaces
Programmer interfaces for embedding DML in programming languages:
1) Pre-compiler Approach 2) Procedure Call Approach
DBMS Interface
User-friendly interfaces provided by a DBMS
1) Menu-based interfaceNo need to memorize the specific commands and syntax of a query language.
2) Graphical interfaceSpecify query via schema diagram and can be combined with menus.
3) Forms-based interfaceUsually programmed for parametric users to fill out the form entries to insert new data for creating canned transactions.
4) Natural language interfaceAccept and interpret requests written in English or some other language.
5) Combination of above
Other DBMS Interface
Interfaces for parametric users (e.g., bank tellers)
Have a small set of operations.
Interface for the DBA
Use function keys for minimizing number of keystrokes.
Use privileged commands for creating accounts, setting system parameters, granting account authorization, changing schema, and reorganizing the storage structure of a database.
Speech as Input and Output
Web Browser as an interface
Database System Environment
Source: Elmasri R. & Navathe S.B. (1994) Fundamentals of
database systems
Database System Environment
1) Stored data manager controls access to DBMS information stored on disk.
2) DDL compiler processes schema definitions, specified in the DDL, and stores descriptions of the schemas (meta-data) in the DBMS catalog. It also compiles commands into object code for database access.
3) Run-time database processor handles database access at run time by executing the request.
4) Query compiler parses and analyzes a query.
5) Precomplier extracts DML commands from an application program written in a host programming language.
DBMS components modules are as follows.
6) DML complier compiles DML commands into object code for database access. The rest of the program is sent to the host language compiler.
Database System Utilities
Common database utilities have the following types of functions
1) Loading existing data files into the database.
2) Backing up copy of the database periodically.
3) Reorganizing database file structures to improve performance.
5) Monitoring database usage and providing statistics to the DBA.
Other functions, such as sorting, user monitoring, data compression, etc.
4) Report generation utilities.
Database System Utilities
Data dictionary is an important and very useful utility.
Used to store schema descriptions and other information such as design decisions, application program descriptions, user information, usage standard, etc.
Active data dictionary is accessed by DBMS s/w and users/DBA.
Passive data dictionary is accessed by users/DBA only.
Combination of catalog/data dictionary:
Data dictionary vs. DBMS catalog
Database Architectures
user
network
Application
Database system
user
Application client
Application server
Database system
network
client
server
Two-tier architecture Three-tier architecture
Source: Silberschatz A., Korth, H.F. & Sudarshan S. (2006) Database system concepts.
Database Architectures
Two Tier Client-Server Architectures
Application on client machine invokes database system functionality at the server machine through query language statements.
Application program interface like ODBC (Open Database Connectivity) and JDBC (Java Database Connectivity) are used for interaction.
Three Tier Client-Server Architectures
Client machine communicates with application server only which means it does not contain any direct database calls.
Application server communicates with a database system to access data.
Appropriate for large applications, and web applications.
Classification of DBMS
Based on the data model used:
Traditional: Relational, Network, Hierarchical
Emerging: Object-oriented, Object-relational
Other classifications
Single-user (typically used with micro-computers) vs. multi-user (most DBMSs).
Centralized (uses a single computer with one database) vs. distributed (uses multiple computers, multiple databases)
Distributed (or client server based database systems, a set of database servers supports a set of clients)
Classification of DBMS
Data model is the main criterion used to classify DBMS.
1) Relational data model represents a collection of tables.
2) Network model represents data as record types and limited type of 1:N relationship, called a set type.
3) Hierarchical model represents data as hierarchical tree structures. Each hierarchy represents a number of related records.
4) Object-oriented model defines a database in terms of objects, their properties, and their operations. Object with the same structure and behavior belongs to a class.
5) Object-relational model combines relational data model and object-oriented model to define complex data types.
Example of Relational Data Model
Source: Elmasri R. & Navathe S.B. (1994) Fundamentals of database systems
STUDENT Name StudentNumber Class Major
Smith 17 1 COSC
Brown 8 2 COSC
COURSE CourseName CourseNumber CreditHours Department
Intro to Computer Science COSC1310 4 COSC
Data Structures COSC3320 4 COSC
Discrete Mathematics MATH2410 3 MATH
Database COSC3380 3 COSC
PREREQUISITE CourseNumber PrerequisiteNumber
COSC3380 COSC3320
COSC3380 MATH2410
COSC3320 COSC1310
Example of Relational Data Model (Cont.)
Source: Elmasri R. & Navathe S.B. (1994) Fundamentals of database systems
SECTION SectionIdentifier CourseNumber Semester Year Instructor
85 MATH2410 Fall 91 King
92 COSC1310 Fall 91 Anderson
102 COSC3320 Fall 92 Knuth
112 MATH2410 Fall 92 Chang
119 COSC1310 Fall 92 Anderson
135 COSC3380 Fall 92 Stone
GRADE_REPORT StudentNumber SectionIdentifier Grade
17 112 B
17 119 C
8 85 A
8 92 A
8 102 B
8 135 A
Example of a Network Schema
Source: Elmasri R. & Navathe S.B. (1994) Fundamentals of database systems
STUDENT
GRADE_REPORT
SECTION
COURSE
PREREQUISITE
STUDENT_GRADES
SECTION_GRADES
COURSE_OFFERINGS
HAS_AIS_A
Example of a Hierarchical Schema
Source: Elmasri R. & Navathe S.B. (1994) Fundamentals of database systems
DNAME DNUMBER MGRNAME MGRSTARTDATE
DEPARTMENT
NAME SSN BDATE ADDRESS
EMPLOYEE
PNAME PNUMBER PLOCATION
PROJECT