Weak Entity Types
Entity
types that do not have key attributes of their own are called weak entity types. In contrast, regular
entity types that do have a key attribute—which include all the examples discussed so far—are called strong entity types. Entities belonging to a weak entity type are
identified by being related to specific entities from another entity type in
combination with one of their attribute values. We call this other entity type
the identifying or owner entity type,10 and we
call the relationship type that relates a weak entity type to its owner the identifying relationship of the weak
entity type.11 A weak entity type always has a total participation constraint (existence dependency) with respect
to its identifying relationship because a weak entity can-not be identified
without an owner entity. However, not every existence dependency results in a
weak entity type. For example, a DRIVER_LICENSE entity
cannot exist unless it is related to a PERSON entity,
even though it has its own key (License_number) and
hence is not a weak entity.
Consider
the entity type DEPENDENT, related
to EMPLOYEE, which is used to keep track of
the dependents of each employee via a 1:N relationship (Figure 7.2). In our
example, the attributes of DEPENDENT are Name (the first name of the
dependent), Birth_date, Sex, and Relationship (to the employee). Two dependents
of two
distinct employees may, by chance, have the same
values for Name, Birth_date, Sex, and Relationship, but
they are still distinct entities. They are identified as distinct entities only after determining the particular employee entity to which each
dependent is related. Each employee entity is said to own the dependent entities that are related to it.
A weak
entity type normally has a partial key,
which is the attribute that can uniquely identify weak entities that are related to the same owner entity.12
In our example, if we assume that no two dependents of the same employee ever
have the same first name, the attribute Name of DEPENDENT is the partial key. In the worst
case, a composite attribute of all the
weak entity’s attributes will be the partial key.
In ER
diagrams, both a weak entity type and its identifying relationship are
distinguished by surrounding their boxes and diamonds with double lines (see
Figure 7.2). The partial key attribute is underlined with a dashed or dotted
line.
Weak
entity types can sometimes be represented as complex (composite, multivalued)
attributes. In the preceding example, we could specify a multivalued attribute Dependents for EMPLOYEE, which is a composite attribute with component
attributes Name, Birth_date, Sex, and Relationship. The
choice of which representation to use is made by the database designer. One
criterion that may be used is to choose the weak entity type representation if
there are many attributes. If the weak entity participates independently in
relationship types other than its identifying relationship type, then it should
not be modeled as a complex attribute.
In
general, any number of levels of weak entity types can be defined; an owner
entity type may itself be a weak entity type. In addition, a weak entity type
may have more than one identifying entity type and an identifying relationship
type of degree higher than two, as we illustrate in Section 7.9.
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