Chapter 3
Relationships in Detail1RelationshipsTen different relationship
typesNontransferabilityRelationships that seem to have
attributesRules of Normalization2OverviewIntroductionThis lesson
discusses in detail how to establish a relationship between two
entities. You meet the ten types of relationship and examples of
the less frequent types. This lesson looks at nontransferable
relationships and discusses the differences and similarities
between relationships and attributes. It also provides a solution
for the situation where a relationship seems to have an attribute.
Finally, the rules of normalization are discussed in the context of
conceptual models.ObjectivesAt the end of this lesson, you should
be able to do the following:Create a well-defined relationship
between entitiesIdentify which relationship types are common and
which are notGive real-life examples of uncommon relationship
typesChoose between using an attribute or a relationship to model
particular informationResolve a m:m relationship into an
intersection entity and two relationshipsResolve other
relationships and know when to do soRules of NormalizationDetermine
the existence of a relationship Choose a name for the relationship
from both perspectivesDetermine optionalityDetermine
degreeDetermine nontransferability
3Establishing a RelationshipDetermining the Existence of a
Relationship Ask, for each of your entities, if it is somehow
related to one or more of the entities in your model, and, if so,
draw a dotted skeleton relationship line.Usually all entities in a
model are related to at least one other entity. Exceptions are
rare, but they do exist.Two entities can be related more than once.
For example, in the Electronic Mail system there are two
relationships between entities MESSAGE and USER, one is about who
is sending a MESSAGE and one about who receives a MESSAGE.An entity
can be related to itself. This is called a recursive relationship.
For example, a MESSAGE can be a reply to another MESSAGE. See the
paragraph on recursive relationships for more details on
this.Establishing the
Relationship4USERMESSAGEsendingreceivingreplyingRelationship
Names5USERMESSAGEsent bysender ofreply ofreplied to by sent
toreceiver ofChoosing a Name for the Relationship Sometimes the
relationship name for the second perspective is simply the passive
tense of the other one, such as is owner of and is owned by.
Sometimes there are distinct words for both concepts, such as
parent of / child of or composed of / part of.Try to use names that
end in a preposition.If you cannot find a name, you may find these
relationship names useful:Consists of / is part ofIs classified as
/ is classification forIs assigned to / is assignment ofIs referred
to / referring toResponsible for / the responsibility ofSometimes a
very short name is sufficient, for example, with, in, of, for, by,
about, at, into.Naming the Relationship6MESSAGEUSERreceivingA
MESSAGE is received by a USERreceived byA USER is receiver of a
MESSAGEreceiver ofNaming the RelationshipAre sent to and receiver
of really opposite? If so, the assumption is that if a MESSAGE is
sent to a USER, it also arrives. Maybe it is safer to name the
relationship received by / receiver
of...Optionality7USERMESSAGEwritten byauthor ofreply ofreplied to
by received byreceiver ofDetermining Optionality of Both the
Relationship EndsAnswer the questions: Must every MESSAGE be sent
by a USER?Must every USER be sender of an MESSAGE?Must every
MESSAGE be sent to a USER?Must every USER be addressed in a
MESSAGE?When an answer is Yes the relationship end is mandatory,
otherwise it is optional.Be careful at this point. Often a
relationship end seems to be mandatory, but actually it is not. In
the ElectronicMail example it seems that every MESSAGE must be sent
by a USER. But a MESSAGE that was sent by an external user to an
internal USER has no relationship to a USER, unless the system were
to keep external users as well.Sometimes a relationship is
ultimately mandatory, but not initially. Such a relationship should
be modeled as optional.Optionality8Must every MESSAGE be received
by a USER?No:Yes:YesMust every USER be receiver of a
MESSAGE?NoMESSAGEreceiver ofUSERreceived byDetermining Degree of
Both the Relationship EndsAnswer the questions:Can a MESSAGE be
written by more than one USER?Can a USER be author of more than one
MESSAGE?If the answer is No the degree is called 1.If the answer is
Yes the degree is called many or just m.This must be determined for
all relationship ends.Note that a mandatory many relationship end
from A to B does not mean that it is mandatory for A to be split
into more than onMandatory 1: Mandatory m9Asplit intopart of BEvery
A must be split into at least one BEvery B must be part of exactly
one ADegree10USERwritten byauthor ofreply ofreplied to by
ATTACHMENTwithcontaining
