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THE ENTITY RELATIONSHIP MODEL

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					                  THE ENTITY RELATIONSHIP MODEL
   The E-R model was introduced by Peter Chen in 1976. Key elements of the E-R model
   are entities, attributes, identifiers and relationships.

 ENTITIES
1. ENTITY TYPE:          An object or concept that is identified by the enterprise as having
   an independent existence. They represent a set of objects in the real world with the
   same properties. Also entity types are named as entity classes.
      Example: Physical existence: Staff, Property, Customer, Part, Supplier, Product,
   etc.
      Conceptual existence: Viewing, Inspection, Sale, Work experience, etc.
             Weak Entity Type: An entity type that is existence-dependent on some
               other entity type. A special weak entity type is ID-dependent entity, in
               which the identifier of one entity includes the identifier of another entity.
             Strong Entity Type: An entity type that is not existence-dependent on
               some other entity type.

2. ENTITY: Definition 1:        Entity is an object or concept that is uniquely identifiable.
           Definition 2:        An entity is something that can be identified in the user’s
                                work environment, something that users want to track.
              Entities are also named as entity occurrence or entity instance.
Examples and diagrammatic        FATHER       MOTHER           CHILD
representation of entity types:

 ATTRIBUTES

ATTRIBUTE: A property of an entity or a relationship type. Also named as property.
  Example: Date_of_Birth, Sex, Age are attributes for Child Entity.
     Attribute Domain is a set of values that may be assigned to a single-valued
      attribute. For example Zip_Code attribute’s domain is 5-digit number for an
      address in Türkiye.
     Simple attribute is an attribute composed of a single component with an
      independent existence. Sex is an example of simple attribute.
     Composite attribute is an attribute composed of multiple components, each
      with an independent existence. Address is an example of composite attribute. It
      can be divided in to Street, Area, City and Zip_Code, which are simple
      attributes. Address can be decided to be a single attribute also. The decision
      depends on the requirements of the model.
     Single-valued attribute is an attribute that holds single value for a single entity.
     Multi-valued attribute is an attribute that holds multiple values for a single
      entity. Example: An entity has multiple values of Tel_No attribute.
     Derived attribute is an attribute that represents a value that is derivable from the
      value of a related attribute or a set of attributes, not necessarily in the same
      entity. Example: Age attribute can be derived from Date_of_Birth attribute.
Examples and diagrammatic representation of Student entity and its attributes:
                                                              StudentName


 StudentNumber                    STUDENT


                                                              StudentYear

 IDENTIFIERS

IDENTIFIER (KEY)
    Key is a data item that allows us to uniquely identify individual occurrences of an
    entity type.
     Candidate Key is an attribute or set of attributes that uniquely identifies
       individual occurrences of an entity type.
     Primary Key is the key that is selected from candidate keys.
       Examples: National_Insurance_Number is unique for every member of stuff.
       Generally, …_ID is primary key for that attribute.
     Composite Key is a candidate key that that consists of two or more attributes.

 RELATIONSHIPS
1. RELATIONSHIP TYPE: A meaningful association among entity types.
2. RELATIONSHIP: An association of entities where the association includes one entity
   from each participating entity type.
3. DEGREE OF A RELATIONSHIP: The number of participating entities in a
   relationship.
                        SALESPERSON                              SALESPERSON
Example of
Binary Relationship                           SP-ORDER
Binary Relationship has a degree of two.


Example of                 MOTHER                                FATHER
Ternary Relationship                                    PARENT
Ternary Relationship has a degree of three.
                                                CHILD

4. CARDINALITY
 Cardinality ratio determines the number of possible relationships for each
   participating entity. The rules defining cardinality are business rules.
 Maximum cardinality shows the maximum number of entities that can be involved in
   a relationship.
 Minimum cardinality shows the minimum number of entities that can be involved in a
   relationship.
 Types of Binary Relationships:
   One-to-one (1:1): A single entity of one type         EMPLOYE      1:1     AUTO
    is related to a single entity of another type.        E
                                                                AUTO ASSIGNMENT


    One-to-many (1:N): A single entity of one type
     is related to many entities of another type. DORMITORY                1:N            STUDENT
                                                      E
                                                                  DORM-OCCUPANT

    Many-to-many (M:N): Many entities of one                              N:M
     type is related to many entities of another type. STUDENT                             CLUB

                                                                      STUDENT-CLUB

5. RECURSIVE RELATIONSHIP: A relationship where                                           1:N
the same entity participates more than once in different roles.     STUDENT


 E-R DIAGRAMS                                                                ROOMS-WITH

Entity-Relationship (E-R) diagrams are used to show the relations between entities.
E-R diagrams show maximum cardinality but
they do not show minimum cardinality. But we      DORMITORY            1:N          STUDENT
can use hash mark for “must” and oval for “free”. E
                                                                  DORM-OCCUPANT

Example of showing attributes in E-R diagrams

                                        Rent
                 DormName                                 StudentNumber


    Location                        1:N
                     DORMITORY                        STUDENT               StudentYear
                     E
                             DORM-OCCUPANT
                Location                                            StudentName
                                       OR

          DORMITORY      1:N                STUDENT
          E
                  DORM-OCCUPANT

DORMITORY Contains               STUDENT Contains          DORM-OCCUPANT Contains

DormName                         StudentNumber             Rent
Location                         StudentName
Number of Rooms                  StudentYear
Multi-value attributes can be represented by creating a new weak entity.

              CUSTOMER            1:N         CONTACTNAME

            CUSTOMER Contains               CONTACTNAME Contains
            CustomerName                        ContactName
            CustomerNumber
            Other attributes…

            Example E-R Diagram!

				
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posted:11/23/2011
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