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Mapping E/R Diagrams to Relational Database Schemas. Second Half of Chapter 3. E/R  Relation Model (Example). since. since. name. name. dname. dname. ssn. did. did. budget. budget. lot. (0,*). (1,1). Departments. Employees. Manages. (0,*). hours_worked. (0,*). Works_In. .

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Mapping E/R Diagrams to Relational Database Schemas

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Mapping e r diagrams to relational database schemas l.jpg

Mapping E/R Diagrams toRelational Database Schemas

Second Half of Chapter 3


E r relation model example l.jpg

E/R  Relation Model (Example)

since

since

name

name

dname

dname

ssn

did

did

budget

budget

lot

(0,*)

(1,1)

Departments

Employees

Manages

(0,*)

hours_worked

(0,*)

Works_In

contractid

hourly_wages

since

Contract_Emps

Hourly_Emps


Logical db design er to relational l.jpg

name

ssn

lot

Employees

Logical DB Design: ER to Relational

1. Entity Types to Tables.

CREATE TABLE Employees

(ssn CHAR(9),

name CHAR(20),

lot INTEGER,

PRIMARY KEY (ssn))


2 relationship types to tables l.jpg

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.


Translating er diagrams with key constraints l.jpg

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)


Review weak entities l.jpg

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

dname

age

ssn

lot

(1,1)

(0,*)

Policy

Dependents

Policy

Dependents

Employees

Parent


Translating weak entity types l.jpg

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)


Review isa hierarchies l.jpg

name

name

ssn

lot

Review: ISA Hierarchies

Employees

hours_worked

contractid

hourly_wages

  • 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)

  • 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.

Contract_Emps

Hourly_Emps


3 translating isa hierarchies to tables l.jpg

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.


Dr eick s default mapping e r relational data model l.jpg

Dr. Eick’s Default MappingE/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.


Slide11 l.jpg

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


Mapping of the multi wedding e r diagram to a relational schema l.jpg

Mapping of the Multi-Wedding E/R Diagram to a Relational Schema

Company(name,location)

Person(ssn,name)

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)


Nfl e r design problem ungraded homework spring 2005 l.jpg

NFL E/R Design ProblemUngraded 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!


Slide14 l.jpg

Sal

empl.

ssn

(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


Using dr eick s e r rel default mapping l.jpg

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)


Slide16 l.jpg

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,*)

  • 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

Another different Solution!


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