1. Mapping E/R Diagrams to Relational Database Schemas
Second Half of Chapter 3
The slides for this text are organized into chapters. This lecture covers Chapter 3.
Chapter 1: Introduction to Database Systems
Chapter 2: The Entity-Relationship Model
Chapter 3: The Relational Model
Chapter 4 (Part A): Relational Algebra
Chapter 4 (Part B): Relational Calculus
Chapter 5: SQL: Queries, Programming, Triggers
Chapter 6: Query-by-Example (QBE)
Chapter 7: Storing Data: Disks and Files
Chapter 8: File Organizations and Indexing
Chapter 9: Tree-Structured Indexing
Chapter 10: Hash-Based Indexing
Chapter 11: External Sorting
Chapter 12 (Part A): Evaluation of Relational Operators
Chapter 12 (Part B): Evaluation of Relational Operators: Other Techniques
Chapter 13: Introduction to Query Optimization
Chapter 14: A Typical Relational Optimizer
Chapter 15: Schema Refinement and Normal Forms
Chapter 16 (Part A): Physical Database Design
Chapter 16 (Part B): Database Tuning
Chapter 17: Security
Chapter 18: Transaction Management Overview
Chapter 19: Concurrency Control
Chapter 20: Crash Recovery
Chapter 21: Parallel and Distributed Databases
Chapter 22: Internet Databases
Chapter 23: Decision Support
Chapter 24: Data Mining
Chapter 25: Object-Database Systems
Chapter 26: Spatial Data Management
Chapter 27: Deductive Databases
Chapter 28: Additional Topics 1

2. E/R  Relation Model (Example)
since
since
name
name
dname
dname
ssn
lot
did
did
budget
budget
(0,*)
(1,1)
Employees
Manages
Departments
(0,*)
hours_worked
(0,*)
Works_In  
hourly_wages
contractid
since
Hourly_Emps
Contract_Emps

3. Logical DB Design: ER to Relational
1. Entity Types to Tables.
Employees
ssn
name
lot
The slides for this text are organized into several modules. Each lecture contains about enough material for a 1.25 hour class period. (The time estimate is very approximate--it will vary with the instructor, and lectures also differ in length; so use this as a rough guideline.) This covers Lectures 1 and 2 (of 6) in Module (5).
Module (1): Introduction (DBMS, Relational Model)
Module (2): Storage and File Organizations (Disks, Buffering, Indexes)
Module (3): Database Concepts (Relational Queries, DDL/ICs, Views and Security)
Module (4): Relational Implementation (Query Evaluation, Optimization)
Module (5): Database Design (ER Model, Normalization, Physical Design, Tuning)
Module (6): Transaction Processing (Concurrency Control, Recovery)
Module (7): Advanced Topics
CREATE TABLE Employees
(ssn CHAR(9),
name CHAR(20),
lot INTEGER,
PRIMARY KEY (ssn)) 3

4. 2. Relationship Types to Tables
CREATE TABLE Works_In(
ssn CHAR(9),
did INTEGER,
since DATE,
PRIMARY KEY (ssn, did),
FOREIGN KEY (ssn)
REFERENCES Employees,
FOREIGN KEY (did)
REFERENCES Departments)
In translating a relationship set to a relation, attributes of the relation must include:
Keys for each participating entity set (as foreign keys).
This set of attributes forms a superkey for the relation.
All descriptive attributes. 5

5. Translating ER Diagrams with Key Constraints
CREATE TABLE Manages(
ssn CHAR(9),
did INTEGER,
since DATE,
PRIMARY KEY (did),
FOREIGN KEY (ssn) REFERENCES Employees,
FOREIGN KEY (did) REFERENCES Departments)
Map relationship to a table:
Note that did is the key now!
Separate tables for Employees and Departments.
Since each department has a unique manager, we could instead combine Manages and Departments.
CREATE TABLE Dept_Mgr(
did INTEGER,
dname CHAR(20),
budget REAL,
manager CHAR(9),
since DATE,
PRIMARY KEY (did),
FOREIGN KEY (manager) REFERENCES Employees) 7

6. Review: Weak Entities
A weak entity can be identified uniquely only by considering the primary key of another (owner) entity.
Owner entity set and weak entity set must participate in a one-to-many relationship set (1 owner, many weak entities).
Weak entity set must have total participation in this identifying relationship set.
name
cost
ssn
dname
lot
age
(0,*)
(1,1)
Policy
Dependents
Employees
Policy
Dependents
Parent 10

7. Translating Weak Entity Types
Weak entity set and identifying relationship set are translated into a single table --- it has a (1,1) cardinality constraint.
CREATE TABLE Dep_Policy (
dname CHAR(20),
age INTEGER,
cost REAL,
parent CHAR(9) NOT NULL,
PRIMARY KEY (parent, dname),
FOREIGN KEY (parent) REFERENCES Employees,
ON DELETE CASCADE) 11

8. Review: ISA Hierarchies
name
name
ssn
lot
Review: ISA Hierarchies
Employees
hours_worked  
hourly_wages
contractid
As in C++, or other PLs, attributes are inherited.
If we declare A ISA B, every A entity is also considered to be a B entity.
Hourly_Emps
Contract_Emps
Overlap constraints: Can Joe be an Hourly_Emps as well as a Contract_Emps entity? (Allowed/disallowed)
Covering constraints: Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no) 12

9. 3. Translating ISA Hierarchies to Tables
General approach:
3 relations: Employees, Hourly_Emps and Contract_Emps.
Hourly_Emps: Every employee is recorded in Employees. For hourly emps, extra info recorded in Hourly_Emps (hourly_wages, hours_worked, ssn); must delete Hourly_Emps tuple if referenced Employees tuple is deleted).
Queries involving all employees easy, those involving just Hourly_Emps require a join to get some attributes.
Alternative: Just Hourly_Emps and Contract_Emps.
Hourly_Emps: ssn, name, lot, hourly_wages, hours_worked.
Each employee must be in one of these two subclasses. 13

10. Dr. Eick’s Default Mapping E/R  Relational Data Model
For each entity type create a relation with the attributes associated with the entity type. Choose a primary key for the defined relation; if the entity type is weak, delay choosing primary keys until all identifying relationships are mapped.
For each relationship type create a relation that contains the roles as well as the attributes of the relationship type. Define referential integrity constraints with respect to the mapped roles. Exception: If there is a (1,1) cardinality constraint do not generate a separate relation, but rather associate the relationship information with the relation of this participating entity type.
For each sub-type create a relation that contains the attributes of the entity type as well as the primary key of the most general super class of this entity type (which also will be the primary key of the generated relation). Define referential integrity constraints with respect to the direct super class of the mapped entity type.

11. E/R Diagram to be mapped
ssn
name
(0,*)
Male 
occurred
husband
Person 
wife 
Female
(0,*)
(1,1)
Wedding
Is-insured
(0,*)
(0,*)
Company
location
Con#
name
amount to
from
E/R Diagram to be mapped

12. Mapping of the Multi-Wedding E/R Diagram to a Relational Schema
Person(ssn,name)
Company(name,location)
Male_Person(ssn)
Female_Person(ssn)
Wedding(husband,wife,from,to)
Is-Insured(hssn,wssn,from,company, amount, Con#)
Correct Syntax: FOREIGN KEY (hssn,wssn,from) REFERENCES
Wedding(husband,wife,from)

13. NFL E/R Design Problem Ungraded Homework Spring 2005
Design an Entity-Relationship Diagram that models the following objects and relationships in the world of football (NFL): teams, players, games, managers and contracts. Each (NFL-) team has a unique team name, and a city it plays in. Each person being part of the NFL-world has a unique ssn and a name. Additionally, for players their weight, height, position and birth dates are of importance. Players have a contract with at most one team and receive a salary for their services, and teams have at least 24 and at most 99 players under contract. Each team has one to three managers; managers can work for at most 4 teams and receive a salary for each of their employments. Players cannot be managers. A game involves a home-team and visiting-team; additionally, the day of the game, and the score of the game are of importance; teams play each other several times in a season (not on the same day!). Moreover, for each game played we like to know which players participated in the game and how many minutes they played.
Indicate the cardinalities for each relationship type; assign roles (role names) to each relationship if there are ambiguities! Use sub-types, if helpful to express constraints!

14. NFL E/R Problem ssn Sal empl. name name Person Team city contr Player
(0,4)
name
name
Manager
isa
(1,3)
Person
Team
isa
city
(24,99)
contr
(0,1)
Player
birthd
Home
Visit
NFL
E/R
Problem
(0,*)
(0,*)
weight
height
pos
Sal
(0,*)
play
played-in.
score
min
(1,1)
(22,*)
Scoring:
Play relationship a Set: 3
Person/Player/Manager: 3
Weak Game Entity: 3
Played-in: 2
Can Only Play once on a day: 1
Contract: 3
Salary, score, min attribute: 3
Game
Date

15. Using Dr. Eick’s E/RRel.-Default Mapping
Employs(team, manager, salary)
Manger(ssn)
Team(name, city)
Player(ssn, birthd, pos,…)
Person(ssn, name)
Game(home, visit, day, score)
Played_in(home,visit, day, ssn, min)

16. Another different Solution!
Sal
empl.
(0,4)
ssn
name
name
Manager
isa
(1,3)
Person
Team
isa
city
(24,99)
contr
(0,1)
Player
birthd
Home
Visit
(0,*)
(0,*)
weight
height
pos
Sal
(0,*)
play
played-in.
Day
(0,*)
score
min
Date
(1,1)
(22,*)
Game
Scoring:
Play relationship a Set: 3
Person/Player/Manager: 3
Weak Game Entity: 3
Played-in: 2
Can Only Play once on a day: 1
Contract: 3
Salary, score, min attribute: 3
Another different Solution!