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الكلية كلية تكنولوجيا المعلومات     القسم قسم البرامجيات     المرحلة 2
أستاذ المادة مازن كاظم حميد المنصوري       05/01/2016 09:10:49
Connectivity and Cardinality
The term connectivity is used to describe the relationship classification.
Cardinality expresses the minimum and maximum number of entity occurrences associated with one occurrence of the related entity in the ERD, cardinality is indicated by placing the appropriate numbers beside the entities, using the format(x, y).The first value represents the minimum number of associated entities, while the second value represents the maximum number of associated entities as in following figure



EXISTANCE DEPENDENCE
An entity is said to be existence –dependent if it can exist in the database only when it is associated with another related entity occurrence.
In implementation terms, an entity is existence –dependent if it has a mandatory foreign key _that is, a foreign key attribute that can not be null.
If an entity can exist a part from one or more related entities, it is said to be existence_ independent .(some times designers refer to such an entity as a strong or regular entity) for example, suppose that the XYZ corporation uses parts to produce its products. farther, suppose that some of those parts are produced in _house and other parts are bought from vendors. In that scenario , it is quite possible for a PART to exist independently from a VENDOR in the relationship "PART is supplied by VENDOR ",because at least some of the parts are not supplied by a vendor . therefore ,PART is existence _independent from VENDOR


RELATIONSHIP STRENGTH
The concept of relationship strength is based on how the primary key of a related entity is defined.
Weak (non-identifying ) relationships
A weak relationship ,also known as a non- identifying relationship, exist if the PK of the related entity does not contain a PK component of the parent entity . by default ,relationships are established by having the PK of the parents entity appear as an FK on the related entity. for example, suppose that COURSCE and CLASS entities are defined as
COURSE(CRS-CODE,DEPT_CODE,CRS_DESCRIPTION, CRS_CREDIT).
CLASS(CLASS-CODE,CRS_CODE,CLASS_SECTION, CLASS_TIME,ROOM_CODE, PROF_NAME).
In this case, a weak relationship exist between COURSE and CLASS because the CLASS_CODE is the CLASS entity s PK, while the CRS_CODE in CLASS is only an FK .
In this example, the CLASS PK did not inherit the PK component from the COURSE entity as figure bellow.


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