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Before beginning to draw the ER
model, read the requirements specification carefully. Document any assumptions
you need to make.
1. Identify entities - list all
potential entity types. These are the object of interest in
the system. It is better to put
too many entities in at this stage and them discard
them later if necessary.
2. Remove duplicate entities -
Ensure that they really separate entity types or just two names for the same
thing.
o Also do not include the system
as an entity type
o e.g. if modelling a library,
the entity types might be books, borrowers, etc.
o The library is the system, thus
should not be an entity type.
3. List the attributes of each
entity (all properties to describe the entity which are relevant to the
application).
o Ensure that the entity types
are really needed.
o are any of them just attributes
of another entity type?
o if so keep them as attributes
and cross them off the entity list.
o Do not have attributes of one
entity as attributes of another entity!
4. Mark the primary keys.
o Which attributes uniquely
identify instances of that entity type?
o This may not be possible for
some weak entities.
5. Define the relationships
o Examine each entity type to see
its relationship to the others.
6. Describe the cardinality and
optionality of the relationships
o Examine the constraints between
participating entities.
7. Remove redundant relationships
o Examine the ER model for
redundant relationships.
ER modelling is an iterative
process, so draw several versions, refining each one
until you are happy with it. Note
that there is no one right answer to the problem, but some solutions are better
than others!
Overview
• construct an ER model
• understand the problems
associated with ER models
• understand the modelling concepts of Enhanced ER
modelling
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