1. Entity-Relationship Diagrams

2. Species Pokémon Moves type level name name status type id Instance Of
Can Learn
Has
Moves
type id

3. Purpose of Entity-Relation Model
The E/R model is a way to roughly describe the system we want to represent in a database
The model represents some of the constraints we want to capture
It may seem silly at first, but design is serious business
These diagrams can help illustrate potential ways to represent your specific system
You can't have an efficient database without good design

4. Entity Set Entity = "thing" or object
Entity Set = collection of similar entities
Akin to a class in object-oriented languages
Attribute = property of (the entities of) an entity set.
Attributes are simple (non-compound values): integers, strings, etc.
You should not have an attribute that is a collection (no lists, structs, sets, etc.)

5. Entity Sets and Attributes
In an Entity-Relationship Diagram:
Entity set = rectangle
Attribute = oval (with a line to the rectangle representing its entity set)
Here, "Species" is the entity set. "type" and "name" are attributes of "Species".
Every entity in "Species" has a "type" and "name".
Species
type
name

6. Relationships A relationship connects two or more entity sets.
Each is represented by a diamond, with lines to the entity sets involved.
Species
Instance Of
Pokémon
type
level
name
name
status

7. Value of an Entity Set
The current "value" of an entity set is the set of entities that belong to it.
Pokémon
name
level
status
aaabaaaajss 69
Sleeping
AIIIIIIRRR 31
NULL
AATTVVV 29
Paralyzed
Species
name
type
Pidgeot
Flying
Lapras
Ice
Venomoth
Poison

8. Value of a Relationship Set
The current "value" of a relationship set is a set of tuples with one component for each related entity set.
Instance Of
Pokémon
Species
aaabaaaajss
Pidgeot
AIIIIIIRRR
Lapras
AATTVVV
Venomoth

9. Multiway Relationships
Relationships can connect more than two entity sets.
Suppose that the relationship set "Battle" involves the entity sets "Pokémon", "Trainers", and "Music".
Battle
level
Trainers
name
Pokémon
name
status
theme
Music

10. Multiplicity of relationships
One-to-One
Many-to-one
Many-to-many
Species
Pokemon
Trainer
has
Instance of
has
Cry
Species
Item

11. What is the multiplicity of character to costume?
Harry
Robe
Gandalf
Jaime
Armor
Cape
Mal
Duster
1. One-to-One
2. One-to-Many
3. Many-to-One
4. Many-to-Many

12. Representing "Multiplicity"
One-to-One relationship has arrows entering both entity sets.
Many-to-One relationship has an arrow entering the "one" side.
Many-to-Many relationship has a line connecting both entity sets.
A Rounded Arrow means "exactly one"
Each entity of the first set is related to exactly one entity of the target set.
Due to limitations of PowerPoint, I am using -> arrows to represent rounded ones and triangle arrows for normal ones.
has
Species
Cry
Each species must have one cry (rounded arrow)
Each Pokémon can have at most one trainer (normal arrow)
has
Pokémon
Trainer

13. What is the multiplicity of Trainer to Catchphrase?
1. Exactly-One-to-Exactly-One
2. One-to-One
3. Many-to-Many
4. Too often
Trainer
says
Catchphrase