1. Multiplicity in E/R Diagrams
Rule: the head of the arrow can have at most one value, given
values to the other entities in the relationship.
Product
Purchase
Store
Person

2. Design Principles What’s wrong? Purchase Product Person President
Country
Person
Moral: be faithful!

3. What’s Wrong? date Product Purchase Store Moral: don’t talk too much.
personAddr
person

4. What’s Wrong? date Dates Product Purchase Store Moral: don’t
complicate life more
than it already is.
Person

5. Do we really need 3-way relationships?
ProductOf
Product
Purchase
StoreOf
Store
Moral:
Find a nice way
to say things.
BuyerOf
Person

6. Modeling Subclasses The world is not flat!
Some objects in a class may have properties not shared by other
members:
Products
Software
products
Educational
products
So --- we define subclasses (in ODL and in E/R).

7. Subclasses in ODL Interface SoftwareProduct: Product{
attribute Set<string> platform;
attribute Set<integer> requiredMemory; }
Interface EducationalProduct: Product{
attribute Struct Interval {integer begin, integer end} ageGroup;
attribute string topic
The two classes also inherit all the properties of Product.

8. Multiple Inheritance Product Platforms required memory ageGroup topic
Educational
Product
Software
Product
Educ-software
Product
Educational-method

9. How do we resolve conflicts?
Rating
(ATA)
Product
Platforms
required memory
ageGroup
topic
Educational
Product
Software
Product
Rating
(ASA)
Educ-software
Product
Educational-method
Rating?

10. Subclasses in E/R Diagrams
name
category
price
Product
isa
isa
Software Product
Educational Product
platforms
Age Group

11. In ODL:
Every object belongs
to a single class
In E/R:
An entity may be
spread out in
multiple sets.
name
category
price
Product
isa
isa
Software Product
Educational Product
platforms
Age Group

12. Modeling Constraints
Extracting constraints is what modeling is all about. But how do
we express them?
Examples:
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.
Domain constraints: peoples’ ages are between 0 and 150.
Why are these constraints useful in the implementation?

13. Keys A set of attributes that uniquely identify an object or entity:
Person: social security number
name
name + address
name + address + age
Perfect keys are often hard to find, so organizations usually
invent something.
An object may have multiple keys:
employee number, social-security number

14. Keys in ODL Interface Person (key ssn) { properties… }
Defining multiple keys:
(key ssn employeID (name address age))

15. Keys in E/R Diagrams name category price Product No formal way
to specify multiple
keys in E/R diagrams
Person
name
ssn
address

16. Single Value Constraints
An entity (or object) may have at most one value for a given
attribute or relationship.
Person: name, social-security number
Company: stock price
How do we do this in ODL?
In E/R, every attribute has at most one value.
Arrows tell us about multiplicity of relations.
If we have a single-valued constraint, we can either:
1. Require that the value exist (see referential integrity shortly)
2. Allow null values.

17. Referential Integrity Constraints
A relationship has one value and the value must exist.
Example:
Product madeBy Company: company must exist.
How do we enforce referential integrity constraints? (otherwise,
we get dangling pointers)
- forbid to delete a reference object, or
- delete the objects that reference an object we’re deleting.
In E/R diagrams:
makes
Product
Company

18. Weak Entity Sets
Entity sets are weak when their key attributes come from other
classes to which they are related.
This happens if:
- part-of hierarchies
- splitting n-ary relations to binary.
affiliation
Team
University
sport
number
name