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Weak entity:
- Dependent is a
weak entity
-
Recall that:
- A weak entity does not have a key
Example:
- Dependent is a
weak entity
-
Weak (= identifying) relationship:
- Every weak entity
in the ER-model
must have
a weak relationship that
provide
identifying information
to the weak entity
Example: the Care-for relationship provides identifying information to Dependent
- Every weak entity
in the ER-model
must have
a weak relationship that
provide
identifying information
to the weak entity
- How to
represent a
weak relationship in
the Relational Model:
- Extend the weak entity relation with the weak relationship
- Example:
- The weak relationship that
we want to
represent is
Care-for between
Employee and
Depedenent:
- The Dependent relation
(right now) is as follows:
Dependent( name, sex, bdate, relationToEmp ) Example content: (Mary, F, 1/1/2000, Daughter) (Jacob. M. 3/4/2001, Son) (Mary, F, 1/1/2000, Daughter)The Care-for relationship is a subset of pairs of the following format:
( Emp-ssn, name (of the dependent) ) Example content: (123-45-6789, Mary) (123-45-6789, Jacob) (222-22-2222, Mary)
- We can represent the
Care-for relationship
by
storing the
Care-for relationship
inside Depedent as
follows:
Dependent( essn, name, sex, bdate, relationToEmp ) Example content: (123-45-6789, Mary, F, 1/1/2000, Daughter) (123-45-6789, Jacob. M. 3/4/2001, Son) (222-22-2222, Mary, F, 1/1/2000, Daughter)
- The weak relationship that
we want to
represent is
Care-for between
Employee and
Depedenent:
- Important note:
- Before adding the
weak relationship to
Depedent:
- The Dependent relation
violated the
Primary Key constraint:
Dependent( name, sex, bdate, relationToEmp )
did not have a key !!!!
- The Dependent relation
violated the
Primary Key constraint:
- After adding the
weak relationship to
Depedent:
- The Dependent relation does
not violate the
Primary Key constraint
any more:
Dependent( essn, name, sex, bdate, relationToEmp )
because essn, name is a key !!!!
- The Dependent relation does
not violate the
Primary Key constraint
any more:
So:
- We just learn how to handle the problem caused by weak entity (without a key) in a database
- Before adding the
weak relationship to
Depedent:
- Summary:
- Our relational model is now:
- Our relational model is now: