LECTURE #17 | WEAK ENTITY TYPE | STRONG ENTITY TYPE | PARTIAL KEY OF A WEAK ENTITY TYPE
NOTES LINK:https://drive.google.com/file/d/1dfeO...
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 cannot 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 3.2). 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
3.2). 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 multivalued composite attribute with the
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 the weak entity type participates
independently in relationship types other than its identifying relationship type.
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 .
10The identifying entity type is also sometimes called the parent entity type or the dominant entity type.
11The weak entity type is also sometimes called the child entity type or the subordinate entity type.
12The partial key is sometimes called the discriminator.