Logical Database Design andEntity-Relationship Mapping

“To Err is Human; to Forgive, Divine.”~ Alexander Pope ~

What exactly is a Logical Model?

• A Blueprint of your database.• It is DBMS independent, and does not rely on

an specific product.• It is a technical communications tool.• Concepts from relational theory are re-

introduced (PK, FK, Table, Column, data type)• Any person knowledgeable in SQL should be

able to read a logical model and create a real database implementation from it.

Concept Map: Conceptual vs. Logical

CONCEPTUAL LOGICAL

ERD Relational Diagram

Entity Relation (Table)

Attribute Column

Relationship Foreign Key (FK)

N/A PK, Constraints

Important: Rows (sample data) MUST be considered in the Logical Model !!!

Relational Notation is Simple

• Table or relation is a box. Name of relation is at the top.

• Columns / Attributes are placed below the relation name in the box.

• Primary Key columns are Underlined and labeled with PK

• Foreign Key columns are labeled with FK

• A PK/FK Relationship between tables are established with an arrow (), which always POINTS TO THE PRIMARY KEY of the relationship.

SUID PKLnameFnameHtWt

Students

CardNo PKSUID FKCardTypeExpDate

CreditCards

Mapping

Mapping is the process of transforming a conceptual model

into a logical model.

Basic Attribute Mapping• Table name is plural• Use Naming

conventions• Assign data types• PK for each table• Break down composite

into simple• Set required to not null• Set Unique and check

constraints

Conceptual

Logical

Mapping 1-M Relationships• FK goes on the

many side• If required on

the one side (aka Weak Entity), then set the FK to not allow null

Conceptual

Logical

Mapping Multi-Valued Attributes

• Similar to 1-M:Place [m] attributes in new table. PK from original table is FK in the new table.C

onceptual

Logical

Mapping Multi-Valued Attributes

• Similar to 1-M:Place [m] attributes in new table. PK from original table is FK in the new table.C

onceptual

Logical

Mapping 1-1 Relationships• Similar to 1-M:

Since the FK goes to the optional side of the relationship, but the FK is set to PK.C

onceptual

Logical

Mapping M-M Relationships

• Make a “bridge table” place FKs in bridge table and set as composite PK

Conceptual

Logical

Mapping Weak Entities

• Make sure the foreign key does not allow null

Conceptual

Logical

Advanced Mapping

Edge cases which are variations on the same themes. Examples provided

for reference.

Ex: Mapping a Unary 1-M RelationshipConceptual

Logical

EmpIDENamePhoneHireDate

Employee

The FK inside the table forms the recursive relationship

Manages

EmpID PKENamePhoneHireDateManagerEmpID FK

Employee

Ex: Mapping a Unary N-M RelationshipConceptual

Logical

ItemNo PKItmNameUnitCost

Item

Bridge Table with both FK’s relating back to the same table PK

ContainsItemNoItmNameUnitCost

Item

ItemNo PK FKComponentItemNo PK FKQty

Component

Ex: Mapping Sub/Super Type

Conceptual Logical

EmpNoENameHireDate

Employee

Discriminator

HourlyRate

EmpHourly

AnnualRateStockOptns

EmpSalary

EmpNo PKENameHireDateEmpType

Employee

EmpNo PK FKHourlyRate

EmpHourly

EmpNo PK FKAnnualRateStockOptns

EmpSalary

Logical Database Design andEntity-Relationship Mapping

Questions?