Lecture 9: Conceptual Database Design
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Lecture 9: Conceptual Database Design January 27 th , 2003
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Lecture 9: Conceptual Database Design. January 27 th , 2003. Building an Application with a DBMS. Requirements modeling (conceptual, pictures) Decide what entities should be part of the application and how they should be linked. Schema design and implementation - PowerPoint PPT Presentation
Transcript of Lecture 9: Conceptual Database Design
Lecture 9:Conceptual Database Design
January 27th, 2003
Building an Application with a DBMS
• Requirements modeling (conceptual, pictures)– Decide what entities should be part of the application and
how they should be linked.
• Schema design and implementation– Decide on a set of tables, attributes.
– Define the tables in the database system.
– Populate database (insert tuples).
• Write application programs using the DBMS– way easier now that the data management is taken care of.
Database Design
• Why do we need it?– Agree on structure of the database before
deciding on a particular implementation.
• Consider issues such as:– What entities to model– How entities are related– What constraints exist in the domain– How to achieve good designs
Database Design Formalisms1. Object Definition Language (ODL):
– Closer in spirit to object-oriented models
– I don’t teach it anymore.
2. Entity/Relationship model (E/R):– More relational in nature.
• Both can be translated (semi-automatically) to relational schemas
• ODL to OO-schema: direct transformation (C++ or Smalltalk based system).
2. Entity / Relationship Diagrams
Entities
Attributes
Relationships between entities
Product
address
buys
Keys in E/R Diagrams
• Every entity set must have a key
Product
name category
price
address name ssn
Person
buys
makes
employs
CompanyProduct
name category
stockprice
name
price
What is a Relation ?
• A mathematical definition:– if A, B are sets, then a relation R is a subset of
A x B
• A={1,2,3}, B={a,b,c,d}, R = {(1,a), (1,c), (3,b)}
- makes is a subset of Product x Company:
1
2
3
a
b
c
d
A=
B=
makes CompanyProduct
Multiplicity of E/R Relations
• one-one:
• many-one
• many-many
123
abcd
123
abcd
123
abcd
address name ssn
Person
buys
makes
employs
CompanyProduct
name category
stockprice
name
price
What doesthis say ?
Multi-way RelationshipsHow do we model a purchase relationship between buyers,products and stores?
Purchase
Product
Person
Store
Can still model as a mathematical set (how ?)
Q: what does the arrow mean ?
A: if I know the store, person, invoice, I know the movie too
Rental
VideoStore
Person
Movie
Invoice
Arrows in Multiway Relationships
Q: what do these arrow mean ?
A: store, person, invoice determines movie and store, invoice, movie determines person
Rental
VideoStore
Person
Movie
Invoice
Arrows in Multiway Relationships
Q: how do I say: “invoice determines store” ?
A: no good way; best approximation:
Q: Why is this incomplete ?
Rental
VideoStore
Person
Movie
Invoice
Arrows in Multiway Relationships
Roles in Relationships
Purchase
What if we need an entity set twice in one relationship?
Product
Person
Store
salesperson buyer
Attributes on Relationships
Purchase
Product
Person
Store
date
Converting Multi-way Relationships to Binary
Purchase
Person
Store
Product
StoreOf
ProductOf
BuyerOf
date
From E/R Diagramsto Relational Schema
• Entity set relation
• Relationship relation
Entity Set to Relation
Product
name category
price
Product(name, category, price)
name category price
gizmo gadgets $19.99
Relationships to Relations
makes CompanyProduct
name category
Stock price
name
Makes(product-name, product-category, company-name, year) Product-name Product-Category Company-name Starting-year
gizmo gadgets gizmoWorks 1963
Start Year
price
(watch out for attribute name conflicts)
Relationships to Relations
makes CompanyProduct
name category
Stock price
name
No need for Makes. Modify Product:
name category price StartYear companyName
gizmo gadgets 19.99 1963 gizmoWorks
Start Year
price
Multi-way Relationships to Relations
Purchase
Product
Person
Storename price
ssn name
name address
Purchase( , , )
3. Design Principles
PurchaseProduct Person
What’s wrong?
President PersonCountry
Moral: be faithful!
Design Principles:What’s Wrong?
Purchase
Product
Store
date
personNamepersonAddr
Moral: pick the right kind of entities.
Design Principles:What’s Wrong?
Purchase
Product
Person
Store
dateDates
Moral: don’t complicate life more than it already is.
Modeling Subclasses
The world is inherently hierarchical. Some entities are special cases of others
• We need a notion of subclass.• This is supported naturally in object-oriented formalisms.
Products
Software products
Educational products
Product
name category
price
isa isa
Educational ProductSoftware Product
Age Groupplatforms
Subclasses in E/R Diagrams
Understanding Subclasses
• Think in terms of records:– Product
– SoftwareProduct
– EducationalProduct
field1
field2
field1
field2
field1
field2
field3
field4field5
Subclasses to Relations
Product
name category
price
isa isa
Educational ProductSoftware Product
Age Groupplatforms
Name Price Category
Gizmo 99 gadget
Camera 49 photo
Toy 39 gadget
Name platforms
Gizmo unix
Name Age Group
Gizmo todler
Toy retired
Product
Sw.Product
Ed.Product
Modeling UnionTypes With Subclasses
FurniturePiece
Person Company
Say: each piece of furniture is owned either by a person, or by a company
Modeling Union Types with Subclasses
Say: each piece of furniture is owned either by a person, or by a company
Solution 1. Acceptable, imperfect (What’s wrong ?)
FurniturePiecePerson Company
ownedByPerson ownedByPerson
Modeling Union Types with Subclasses
Solution 2: better, more laborious
isa
FurniturePiece
Person CompanyownedBy
Owner
isa
Constraints in E/R Diagrams
Finding constraints is part of the modeling process. Commonly used constraints:
Keys: social security number uniquely identifies a person.
Single-value constraints: a person can have only one father.
Referential integrity constraints: if you work for a company, it must exist in the database.
Other constraints: peoples’ ages are between 0 and 150.
Keys in E/R Diagrams
address name ssn
Person
Product
name category
price
No formal way to specify multiple keys in E/R diagrams
Underline:
Single Value Constraints
makes
makes
v. s.
Referential Integrity Constraints
CompanyProduct makes
CompanyProduct makes
Other Constraints
CompanyProduct makes<100
What does this mean ?
Weak Entity SetsEntity sets are weak when their key comes from otherclasses to which they are related.
UniversityTeam affiliation
numbersport name
Handling Weak Entity Sets
UniversityTeam affiliation
numbersport name
Convert to a relational schema (in class)
