1. ER to Schema

2. From E/R Diagrams to Relations
Entity Set -> Relation.
Attributes -> Attributes.
Relationships -> Relations whose attributes are only:
The keys of the connected entity sets.
Attributes of the relationship itself.

3. Entity Set -> Relation
Pokémon(name, species)
name
Pokémon
species

4. Relationship -> Relation
Pokémon(name, species)
Trainer(name)
Belongs(p_name, t_name)
Fought(attacker, defender)
name
name
belongs
Trainer
defender
Pokémon
attacker
species
fought

5. Combining Relations You can "combine" relations together:
Pokémon(name, species) + Belongs(p_name, t_name) becomes:
Pokémon(name, species, trainer)
Rule: You can combine in the relationship sets with an entity set, if that set is the "many" part of a many- to-one relationship.

6. Can we combine Trainers and Fought?
1. No, cyclical relationships sets can't be merged
2. Yes, yielding Trainer(name, badges, defender)
3. No, the "fought" relation is not many- to-one
4. The Pokémon references are silly
name
badges
Trainer
defender
attacker
fought

7. Many-to-Many Combination
Combining "Fought" with "Trainers" leads to redundancy.
Trainers
Fought
Combined
name
badges
Ash 8
Brock 2
Misty 3
Oak
attacker
defender
Ash
Brock
Misty
name
badges
defender
Ash 8
Brock 2
Misty 3
NULL
Oak
Redundancy! See badges.

8. Weak Entity Sets to Relations
Relation for a weak entity set must include attributes for its complete key (including those belonging to other entity sets), as well as its own, non-key attributes.
A supporting relationship is redundant and yields no relation (unless it has attributes).
Relations:
Pokémon(name, position)
Status(effect, duration, position)
Has(effect, position) Redundant
position
name
has
Pokémon
Status
effect
duration

9. Subclasses: 3 methods
Nulls: One relation; entities have NULL in attributes that don't belong to them
Object-oriented: One relation per subset of subclasses, with all relevant attributes
E/R style: One relation for each subclass:
Key attributes
Attributes of that subclass

10. Example: Nulls isa isa name type Pokémon name type owner run_away Abra
psychic
NULL
True
Pikachu
electric
Red
False
Onix
rock
Brock
Mew
Pokémon
isa
isa
owner
Caught Pokémon
Wild Pokémon
run_away

11. Example: Object-oriented
Pokémon
name
type
name
type
Mew
psychic
Pokémon
Wild Pokémon
name
type
run_away
Abra
psychic
True
isa
isa
Caught Pokémon
name
type
owner
Onix
rock
Brock
Caught and Wild Pokémon
owner
Caught Pokémon
Wild Pokémon
run_away
name
type
owner
run_away
Pikachu
electric
Red
False

12. Example: E/R style isa isa name type Pokémon Caught Pokémon name type
Abra
psychic
Pikachu
electric
Onix
rock
Mew
name
owner
Onix
Brock
Pikachu
Red
Pokémon
isa
isa
Wild Pokémon
name
run_away
Abra
True
Pikachu
False
owner
Caught Pokémon
Wild Pokémon
run_away

13. Which method to handle subclasses is best?
1. Object-oriented 2. Nulls 3. E/R style 4. Depends