INFS1000 Lecture Notes - Lecture 5: Associative Entity

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WEEK 5: DATA (ERD) MODELLING
CREATING ERDS
1. Identify entities
a) Business objects you need to store information about
b) Irrelevant entities:
§ Nouns that can be calculated e.g. revenue
§ Nouns that are actually attributes e.g. name, address, age
§ Nouns that have one instance
2. Identify business rules
b) Find ‘NOUN verb NOUN’ relationships in narrative
3. Define relationships and represent cardinality
c) Some business objects are actually relationships (associative entity)
4. Identify attributes
d) Characteristics of entities, represent info you want to store about entities
Example: The Faculty of Vet Science employs 44 academics, all who work in one of five research centres. Many are
also members in at least one research group, which investigate a range of research topics. Research groups do not
overlap in their research interests. Each academic can be mentored by another academic.
o Business rules to identify relationships:
NOUN
Verb
NOUN
ACADEMIC
works in
CENTRE
ACADEMIC
is a member of
RESEARCH GROUP
RESEARCH GROUP
investigates
RESEARCH TOPIC
ACADEMIC
mentors
ACADEMIC
EXTENSIONS IN ERD’S
Unary Relationships: relationship b/w two different instances of an entity
o E.g. when one employee supervises other employees
The Transaction (M:M)
o Modelled with an associative entity e.g. booking, order
o Extend composite primary key (e.g. with time) to allow for multiple transactions b/w the same subject and
object (e.g. hotel guest and room) over time
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Document Summary

Identify entities: business objects you need to store information about. Nouns that can be calculated e. g. revenue. Nouns that are actually attributes e. g. name, address, age. Identify business rules: find noun verb noun" relationships in narrative, define relationships and represent cardinality, some business objects are actually relationships (associative entity) Identify attributes: characteristics of entities, represent info you want to store about entities. Example: the faculty of vet science employs 44 academics, all who work in one of five research centres. Many are also members in at least one research group, which investigate a range of research topics. Research groups do not overlap in their research interests. Each academic can be mentored by another academic: business rules to identify relationships: Academic works in is a member of investigates mentors. Extensions in erd"s: unary relationships: relationship b/w two different instances of an entity, e. g. when one employee supervises other employees.

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