Logical DB Design 5. 1 CSE2132 Database Systems Week 5 Lecture Logical Database Design.

Logical DB Design 5. 1

CSE2132 Database Systems

Week 5 Lecture

Logical Database Design


Logical DB Design in the System LifecycleProject Identificationand Selection

Project Initiationand Planning

Logical design

Analysis

Physical design

Implementation

Maintenance

Conceptual Data Modeling

Enterprise Modeling


Physical Database Design

Database Implementation

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DB Maintenance



Performed by Data Analysts and Database Administrators

Steps

1. Conceptual Model (ER Diagram) mapped onto a logical model dependent on the DBMS characteristics. (The group of slides following numbered 1 to 8)

2. De-normalization (Optimize for efficiency). (The remaining slides in the lecture)


Logical Database Design• Inputs

– Conceptual Model (E-R Diagram ).

– Operational Requirements ( Response, Security etc. )

– Volume and Usage Quantification.

– Consistency Constraints(Referential and User Integrity).

– High Level Program Specification.

– DBMS Characteristics.

• Outputs– DBMS Processable Schema.

– Subschemas.

– Specifications for Physical Design.

– Program Design Guidelines.

– Guide to Database Operations.


E-R Diagram to Relational Model

*Convert the ER Diagram to Tables

* Transform the Attributes in the ER Diagram to columns

* Identify the Primary Keys

* Identify the Referential Constraints

Overview


EMPLOYEEDEPARTMENT

PROJECTSUPERVISION

DEPENDENT

DEPENDENTS_OF

WORKS_ON

MANAGES

WORKS_FOR

CONTROLS

N 1

11

1 N 1

N

NM

Fname Lname

NameSex

AddressSalary

Snn

Bdate

StartDate

NameNumber

Locations

Hours

NameNumber

Location

Name Sex Birthdate

1

N

E/R Diagram


ENTITY TABLE PRIMARY KEY

Employee Employee Employee.SSNDepartment Department Department.NumberProject Project Project.Number

1. Each Entity becomes a Table


For each weak entity create a table- include the Primary Key attribute of the owner tables The Primary Key becomes the :-

. owner key plus the weak entity key

Entity Table Primary Key

Dependent Dependent Employee.SSN + Dependent.Name

2. Weak Entities become Tables


For each binary 1:1 relationship choose the entity withtotal participation and include the relationship attributes in that entity

EMPLOYEE MANAGES DEPARTMENT

- departments must always have a manager- employees are not always managers

Department has mandatory participation in the relationship.

- include the attributes of MANAGES (Start Date) and the primary key of EMPLOYEE (SSN) in the DEPARTMENT entity ( named so as to indicate role Mgrssn )

3. One to One Relationships


For each 1:N relationship - float the PK from the from theentity on the 1 side to the entity on the N(many) side where itwill become a foreign key. If the relationship has any attributes they will also float to the N(many) side.

EMPLOYEE WORKS_FOR DEPARTMENT

N 1

Department Number is moved into the Employee entity

4. 1:N Relationships - Float the PK


For each M:N relationship create a new table with the Primary key being the the PK of both entities involvedin the relationship plus any attributes of the relationship.

EMPLOYEE WORKS_ON PROJECT E# HOURS P#

E1 P1 3E1 P2 4E2 P1 5E3 P2 3

5. M:N Relationships


For each multivalued attribute create a new relation.The Primary Key is the PK of the entity plus the multivalued attribute.

DEPARTMENT LOCATION

TABLE PRIMARY KEY

DEPARTMENT_LOCATIONS DEPARTMENT LOCATION

D1 MELB D1 SYD D2 MELB

6. Multivalued Attributes


For N-ary ( more than binary) Relationships create a new entity and float the Primary Key of each entity involved inthe relationship to the new entity.

Supplier supplies Parts from Cities

TABLES PRIMARY KEY

SUPPLIER_PARTS_CITIES S# P# C# S1 NUT MELB S1 NUT SYD

7. N-ary Relationships


Fname Lname SSN Bdate Address Dno Salary Superssn Sex

Dname Dnumber Mgrssn Mgrstartdate

Dnumber Dlocation

Essn Pno Hours

Pname Pnumber Plocation Dno

Essn Dependent_name Sex Bdate

EMPLOYEE (100)

DEPARTMENT(10)

DEPT_LOCATIONS (15)

WORKS_ON(500)PROJECT (20)

DEPENDENT (200)

p.k

f.k

p.k

f.k f.k

f.k

NOTNULL

NOTNULL

f.kf.k

f.k

f.k

8. Include Volumes for each Tablein the Data Structure Diagram


Delete/Update Restricted/Nullifies/Cascades

Setting the Referential Integrity Constraint Actionscan be performed as follows:-

. if the Foreign Key is part of PK then you should useCASCADES - ESSN is part of PK in WORKS_ON

. if the participation constraint is total( mandatory) - use RESTRICTED - MGRSSN (Depts must have a MGR)

. if the participation constraint is partial(optional) - use NULLIFIES DNO in EMPLOYEE

Nb: These are guidelines and the choice depends on thebusiness rules.

Referential Integrity


Optimisation - maximise/minimise a strength/weakness of a resource

Therefore, we must know the characteristics of resources - cost/performance

List expensive resourcesList cheap resources

Optimize the Logical Model


Application Development Time

Execution Time of Transactions

Data Storage Size

Flexibility to Changing requirements

. trade-off between the above depending on the priorities of the user (User Level Agreement)

Measure Success Optimization Against


- Time to develop a system is kept to a minimum if the design is simple - ensuring no complex relationships - no tricks

- must have staff who are familiar with products anduse Rapid Application Development techniques

Optimisation of Development Time


Reduce the amount of energy used against critical paths(Translates to : - reduce the amount of work againstaccess paths)

Use Indexes

Use Controlled Redundancy Add control Fields(Derived Columns) Add Key Data Elemental Redundancy

Split Like Fields due to Unlike Requirements (Table Splitting)

Optimisation of Execution Time


CUSTOMER

ORDER

Tradeoff against flexibility - waste space, extra maintenance,more programming, more program work

> 20% redundancy - design problems

CUSTOMER

C#, CName, CAddress, CStatus

ORDER

O#, C#, OTotal, ODate

Redundancy - General Comments


CUSTOMER

C#, CName, CAddress, CStatus, CBalance

ORDER


- totals are stored rather than calculated at query time

Add Control Fields(Derived Columns)


This may be characterized as moving information from the child table to the parent table.

CUSTOMER

C#, CName, CAddress, CStatus, Last_O#, Last_ODate

ORDER


- this will allow us to bring back the most recent orderand answer queries such as “What was the last date Customer Smith made an order?”

Add Key Data


This may be characterized as moving information from the parent table to the child table.

CUSTOMER

C#, CName, CAddress, CStatus

ORDER

O#, C#, OTotal, ODate, Caddress

- when we go to print out the Invoice we will not need togo to the Customer file to get the address

Elemental Redundancy


The table is split into two tables. The attributes (or fields) have quite different usage patterns.

CUSTOMER

C#, CName, CAddress

CUSTOMER BATCH

C#, CStatus

- the status is only used by the batch program “update status”. So removing it makes the record smaller thus more records fit on a page of memory and less physical I/O is required to process each of the files.

Vertically Partition a Table


Usually optimizing for efficiency is more important than optimizing for storage.

EMP(E# C7, EAddress C60, EQualification C40) - 107 bytes

1000 emp x 107 bytes = 107000 bytes

Using a CODE TABLE

EMP(E# C7, EAddress C60, EQual_Code C3) - 70 bytes

1000 x 70 = 70000 bytes

QUAL(Qual_Code C3, Qual_Desc 40) 43 x 500 = 21500 bytes

Creation of a Table to Optimize Storage


Optimization of FlexibilityDo not de-normalize (ie. do not introduce redundancy).

Choose a data model which maximises data

independence.

Create an elastic data structure i.e. one which

is responsive to changing requirements

e.g. rather than hardcode values in a program

place the codes in a database.


Physical Design• Stored Record Design

• Choice of File Organization

• Indexes and Clustering

• Choice of Compression

• File Placement

Logical DB Design 5. 1 CSE2132 Database Systems Week 5 Lecture Logical Database Design.

Documents

logical db design

department entity

logical model dependent

entity employee

employee entity

db maintenance slide

program design guidelines

n er diagram slide