Database DesignChapter-2- Database System Concepts and ArchitectureReference: Prof. Mona Mursi Lecture notesDr.Lilac Safadi Lecture noteshttp://www.cse.mrt.ac.lk

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Architecture EvolutionDBMS was a tightly integrated systemDBMS now is modular in designwith client- server architectureMainframes replaced by hundredsof distributed computers connected by a network

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ClientServer ArchitectureClient 1Client 2LANServer(DBMS)DatabaseClient Module Run on user computer Application programs & user interface that access DB Handles user interaction and supports GUIServer Module Handles data storage. Access, search .. ect

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Data ModelData Abstraction : hiding details of data storage that are not needed by most DB users.Data Model: a collection of concepts used to describe the structure of the DB and a set of operations for specifying retrieval and updates of the DB.DB Structure :Data typesRelationshipsConstraints

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Categories of Data ModelsHigh Level (Conceptual Data Model)Representational (implementation Data Model)Low Level (Physical Data Model)Provides concepts close to the wayMany users see dataProvides concepts may be understood by end users but are not far from the way the data is organized.(Entities, Attributes, Relationships)Relational Data Model widely usedDetails of how data is stored in the computer. (Computer specialist)(Records format, ordering, access paths : makes the search for a record more efficient)

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Representational (implementation Data Model)Entity: Represent a real-world objects or concepts. (ex: employee)Attribute: Represent some property of interest that describes an entity. (ex: salary, name)Relationship: among two or more entities. (ex: a works-on relationship between an employee and a project)

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Database Schema

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What is DB Schema?A description of the database (meta-data)Covers only some aspectsE.g. name of record type, data items, some constraintsNot changed frequentlySpecify when a new database definedMeta-data = DBMS Catalog

Schema constructs: Objects in schemaSchema diagram : Convention to display the structure of each record but not the actual instance

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Schema Diagram STUDENTCOURSEName Student_Number Class MajorC_Name C_Number CreditHours DepartmentNote:Schema diagram displays only some aspects of a schema.Ex: does not show the data types nor the relationship between files.Some conditions can not be represented . EX: students majoring in computer science must take CS1310 before the end of their sophomore year.Schema Construct

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Instances and SchemaSchemaInstancesThe data storedIn the database at any given timeThe overall designOf the DB

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Example: University DatabaseDB SchemaA DB InstanceSchema Constructs

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Three Levels of Database AbstractionPhysical LevelConceptual LevelView 1View 2View 3View N..Physical Schema(files of records or structs) ONLY ONE SCHEMAConceptual SchemaEx: name, account, balanceONLY ONE SCHEMA

Subschema Part of the schemaControls what info can be seenAS MANY AS NEEDED

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Database StateDatabase state : the data in the database at particular moment in time. (current set of occurrences or instances).Every insert, delete, update will change the database state.Important difference between database state and database schema.Define a new DB => specify Database schema to the DBMS, At this point the database state is empty state. Database is populated or loaded with data => database state changes. At any time there is a current database state.

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Database StateValid state: every state of the database satisfies the structure and constraints specified in the schema.The DBMS stores the structure and constraints (meta- data) in the DBMS catalog.

Schema Evolution : adding another data item to the schema. Ex: adding dateofbirth to the student schema.

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Database InstancesData in the database at a particular time

Instance = snapshot = database state ~= current set of occurrences DB_state(i) insert/delete/update DB_state(i+1)

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Schema, Instances & DB StatesDefine DB schema DB_state(0) = empty stateDB_state(0) populate initial stateDB_state(i) is a valid state for all ISatisfies the structure and the constraints defined in the schema

DB Schema = intention of the databaseDB state = extension of the Schema

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Three - Schema Architecture

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DBMS Architecture and Data IndependenceCharacteristics of database approach:Insulation of program and data (program-data and program-operation independence)Support of multiple user viewsUse of catalog To achieve the previous characteristics => three schema architecture was proposed.The goal was to separate the user applications and the physical database.

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Visualization of Three-Schema Architecture

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Person 2

text

Data

USER 1

USER 2

EXTERNAL VIEW 1

EXTERNAL VIEW 2

CONCEPTUAL SCHEMA

INTERNAL SCHEMA

END USER

. . .

EXTERNAL LEVEL

CONCEPTUAL LEVEL

INTERNAL LEVEL

Conceptual/internal mapping

External/conceptual mapping

STORED DATABASE

Three-Schema ArchitectureInternal level: describes how data is stored. The way perceived by the DBMS & OS.

Conceptual level: describes what data stored in DB, and the relationships among the data. The way perceived by the DBA & programmers.

External level: describes the part of the DB that user is interested in. The way perceived by the end users.

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Three-Schema Architecture

Emp_No FName LName Dept_NoStaff_No LName SalaryView 1View 2InternalLevelConceptualLevelEMPLOYEEEmp_NoCHAR(6)FNameCHAR(15)LNameCHAR(15)Dept_NoCHAR(3)SalaryNUMBER(5)

PREFIXTYPE=BYTE(6),OFFSET=0EMP#TYPE=BYTE(6),OFFSET=6, INDEX=EMPXLNM TYPE=BYTE(15),OFFSET=12FNM TYPE=BYTE(15),OFFSET=27DPT# TYPE=BYTE(4),OFFSET=42PAY TYPE=FULLWORD,OFFSET=46ExternalLevel

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MappingMapping is the process of transforming requests and results between the Internal, Conceptual & External levels.

Two types of mapping:- External / Conceptual mapping-Conceptual / Internal mapping

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DBMS & Three- Schema ArchitectureNot all DBMS support three-schema architecture because: the mapping between the levels require the catalog to be expanded to include information on how to map requests and data among the levels.=> this requires a software to accomplish the mapping.=>may cause overhead during compilation or execution of a query or a program & time consuming.

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Data Independence

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Data IndependenceData independence: is the ability to modify a schema definition in one level without affecting a schema definition in the next higher level.

Two levels of data independence:Logical data independencePhysical data independence

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Data IndependenceInternalSchemaConceptualSchemaExternalSchemaExternalSchemaExternalSchemaExternal/ConceptualMappingConceptual/InternalMappingLogical data IndependencePhysical data Independence

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1- Logical data independenceThe ability to change the conceptual schema without affecting the external schemas. (application programs)Ex: adding/ removing a record or a data itemThe external schema that refer to the remaining data should not be affected.

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2- Physical data independenceThe ability to change the internal schema without affecting the conceptual or external schemas. This change might be for reorganizing the physical files. EX: creating new access structures to improve the performance of retrieval or update.( i.e. change in the physical files does not affect the queries)

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Database Languages

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Purpose of a Database LanguageDDL (Data Definition Languages)Conceptual schemaInternal schemas (when schema separation is not clear)External schema (in most DBMSs)SDL (Store Definition Languages)Internal schemaVDL (View Definition Languages)External viewDML (Data Manipulation Languages)To manipulate the populated dataModern DB All in one (E.g. SQL)Separate SDL

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Data Definition Language (DDL)DDL is a descriptive language for defining the database schema

DDL compiler generates meta-data stored in a the data dictionary

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Data Manipulation Language (DML)DML is a language for retrieving and updating (insert, delete, & modify) the data in the DB

Two classes of languages Procedural: user specifies what data is required and how to get those data

Nonprocedural: user specifies what data is required without specifying how to get those data (e.g. SQL)

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Database LanguagesDBMS have a facility for embedding DDL & DML (sub-languages) in a High-Level Language (COBOL,PASCAL, C), which in this case is considered a host language.DBMSCall to DBC,C++,Lisp,..Local Vars(memory)ApplicationProgram

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Where the DML is Used?When DML is embedded in programProgramming Language host languageDML data sub languageFor interactive queriesDML query language

General usersUser friendly interfaces

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DBMS InterfacesMenu- basedForm- basedGraphic user interfaceNatural Language interfaceInterface for parametric usersInterfaces for DBA

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The Database System Environment

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Software Components of a DBMSDBMS Component ModulesDatabase System UtilitiesToolsApplication Environments

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DBMS Component Modules (1/2)

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Data

COMPILED (CANNED) TRANSACTIONS

PRIVILEGED COMMANDS

APPLICATION PROGRAMS

INTERACTIVE QUERY

DML STATEMENTS

Application programmers

Casual users

DDL STATEMENTS

DBA Staff

Precompiled

Host Language Compiler

Query Compiler

System Catalog/Data Dictionary

DDL Compiler

DML Compiler

Run-time Database Processor

Stored Data Manager

Concurrency Control/Backup/Recovery Subsystems

STORED DATABASE

execution

execution

Parametric users

execution

DBMS Component Modules (2/2)Disk access control OS:DB & catalog are stored on disk .Access to disk is controlled by OS which schedules disk input/output.Stored Data Manager: controls access to DBMS information stored in the disk.CompilersDDL Compiler : process schema definition specified in the DDL, and stores the description of the schema (meta-data) in the DBMS catalog.Query Compiler: handles high- level queries that are entered interactively.DML CompilerPrecompiler /Host language compiler: extracts DML commands from an application program written in a host programming language. These commands are sent to the DML complier for compilation into object code for database access.Handling DB access at runtimeRuntime database processor

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Database System UtilitiesLoading: load/transfer existing data files (e.g. text files or sequential files) into the database.Backup: backup copy of the database by dumping the entire databases onto tape.File reorganization: to improve performancePerformance Monitoring: monitors database usage and provides statistics to the DBA.

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ToolsCASE( Computer Aided Software Engineering Tools): used in the design phase of database system. Used by DB designers/DBA/usersExamples:Oracles Designer/DeveloperPowersofts S- Designer tool.Information Engineering Facility (IEF) to implement the ER.Data repository systems (Information repository)Used in large organizations, accessed directly by users or DBAStores : Catalog information (schema, constraints).Design decisions, Usage standards , application program descriptions and user information

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Application Development EnvironmentsE.g. PowerBuilder/JBuilderSupportsDevelopment of database applicationsDatabase designGUI developmentQuerying and updating Application program development

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DBMS Classifications

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DBMS Classification Based on Data ModelRelational data modelHierarchical data modelNetwork data modelExample of network data model

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DBMS Classification Based on Number of Sites (view of control)

Centralized systemsDistributed DBMSs (DDBMS)Homogeneous DDBMSHeterogeneous DBMS (Federated DBMS/multi-database systems)

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Chapter -2- SummaryData ModelDatabase schemaDatabase state, instancesThree Schema ArchitectureMapping Data IndependenceDatabase Languages & InterfacesThe Database System EnvironmentDBMS Classifications

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