Conceptual to Logical Modelling

Question 1

what are the goals (3)

Answer 1

preserve all information

maintain constraints as much as possible

minimise null values

Question 2

what constraints cant we represent

Answer 2

1:10 cardinality

exhaustive subtypes

Question 3

what are the nine steps

Answer 3

1. strong entities
2. weak entities
3. 1:1
4. 1:N
5. M:N
6. multivalued
7. Nary
8. disjoint/overlap
9. union

Question 4

how do we map weak entites

Answer 4

create a relation

PK= partial key and owner key

Question 5

how do we map 1:1 relationships (3 methods)

Answer 5

FK approach- include as FK

merged relation approach- merge the two entities into a single relation

relationship relation approach - create a new relation

Question 6

how do we map 1:N relationships

Answer 6

include as FK in the N side, the PK of the 1 side

Question 7

how do we map M:N relationships

Answer 7

create relation for relationship

PK is the composite of the two keys

Question 8

what are the 4 methods to map specialisation and generalisation

what types are they suited for

Answer 8

multiple relations- sub and super (all 4)
create a relation for all entities using the key from super

multiple relations- subs only (total and disjoint)
create relations for subclasses using PK from super and include all supers attributes

single relation- one type attribute (disjoint)
create relation for super and include attributes for each type of sub

single relation- many type attribute (overlap and disjoint)
create relation for super containing its own and sub attributes

Question 9

how do we map union types

Answer 9

include supers PK and FK