The Entity Relationship Modeling

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Module: Database Systems Chapter 1 The Entity Relationship Modelling The Concepts of Entity Relationship Model 1.1 Entity Types
  A group of objects with the same properties, which are identified by the enterprise as having an independent existence. Entity occurrence is referring to a unique identifiable object of an entity type.

Figure 1.0 Example of entities with a physical or conceptual existence

1.2

Relationship Type
  A set of meaningful associations among entity types. It is a set of associations between one or more participating entity types. Each relationship type is given a name that describes its function.

1.3

Relationship Occurrence
  It indicates the particular entity occurrences that are related. Consider a relationship type called ‘has’, which represents an association between branch and staff entity. Each occurrence of the ‘has’ relationship associates one branch entity occurrence with one staff entity occurrence.

Figure 1.1 A semantic net showing individual occurrences of the has relationship type

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1.4

Relationship Types
 A relationship of degree two is called binary.

Figure 1.2 An example of a binary relationship

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A relationship of degree three is called ternary.

Figure 1.3 An example of a ternary relationship called registers

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A relationship of degree four is called quaternary.

Figure 1.4 An example of a quaternary relationship called arranges

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A Recursive relationship is a relationship type where the same entity type participates more than once in different roles.

Figure 1.5 An example of a recursive relationship called supervises with role names supervisor and supervisee

1.5

Attributes
         A property of an entity or a relationship type. For example, a staff entity type may be described by the staffno, name, position and salary attributes. The attributes hold values that describe each entity occurrence and represent the main