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Entity-Relationship Model

Jae H. Park 2006. 2. Entity, Relationship, Attribute - Definition. EntityAnything (such as a person, place, thing, or event) about which data are to be collected and storedCould be physical objects (customer, product, student, etc.) or abstraction (enrollment, flight route, order, etc.) Relatio

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Entity-Relationship Model

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    1. Entity-Relationship Model BEDU 496, Records Administration Using Database Jan. 30, 2006 Dr. Jae Park, jae.park@emich.edu STS, Eastern Michigan University

    2. © Jae H. Park 2006 2 Entity, Relationship, Attribute - Definition Entity Anything (such as a person, place, thing, or event) about which data are to be collected and stored Could be physical objects (customer, product, student, etc.) or abstraction (enrollment, flight route, order, etc.) Relationship Association among (two or more) entities Cardinality: One-to-one, one-to-many, many-to-many relationships Attributes Characteristics of an entity Student’s (entity) attributes: student ID, student name, address, etc.

    3. © Jae H. Park 2006 3 E-R Model Entity-Relationship model is a detailed, logical representation of the data for an organization or for a business area. E-R model is expressed in terms of Entities in the business environment, The relationships among those entities, and Attributes of both the entities and their relationships E-R model is normally expressed as an Entity-Relationship Diagram (or ERD), which is a graphical representation of an E-R model

    4. © Jae H. Park 2006 4 E-R Model Notation

    5. © Jae H. Park 2006 5 E-R Model Relationship Degree

    6. © Jae H. Park 2006 6 E-R Model Relationship Cardinality

    7. © Jae H. Park 2006 7 Entities Entities A person, place, object, event, or concept in the user environment about which the organization wishes to maintain data Entity type A collection of entities that share common properties or characteristics Person (Employee, student, patient) Place (Store, warehouse, state) Object (Machine, building, automobile) Event (Sale, renewal, order, payment, enrollment) Concept (Account, course, membersip) Entity instance A single occurrence of an entity type

    8. © Jae H. Park 2006 8 Strong Entity vs. Weak Entity Strong Entity Type An entity that exists independently of other entity types Weak Entity Type An entity type whose existence depends on some other entity type Identifying owner or owner The entity type on which the weak entity type depends Identifying relationship The relationship between a weak entity type and its owner

    9. © Jae H. Park 2006 9 Strong Entity vs. Weak Entity

    10. © Jae H. Park 2006 10 Strong Entity vs. Weak Entity

    11. © Jae H. Park 2006 11 Attribute A property or characteristic of an entity type that is of interest to the organization Can be associated with only one entity type or relationship Naming Use a noun name Use initial capital letter followed by lower case letters If more than two words, use underscore character to connect the words and start each with a capital letter (Employee_Name)

    12. © Jae H. Park 2006 12 Required vs. Optional Attributes Required Attribute An attribute of an entity that must have a value for each entity instance Optional Attribute An attribute of an entity that may not have a value for every entity instance An attribute without a value is said to be null

    13. © Jae H. Park 2006 13 Simple vs. Composite Attributes Simple attribute An attribute that cannot be broken down into smaller components Composite attribute An attribute that can be broken down into component parts

    14. © Jae H. Park 2006 14 Single-valued vs. Multivalued Attributes Multivalued attribute An attribute that may take on more than one value for a given entity instance E.g., Programming skills like C, Java, Visual Basic, etc.

    15. © Jae H. Park 2006 15 Stored vs. Derived Attributes Derived Attribute An attribute whose values can be calculated from related attribute values

    16. © Jae H. Park 2006 16 Identifier An attribute or combination of attributes that uniquely identifies individual instances of an entity type

    17. © Jae H. Park 2006 17 Composite Identifier An identifier that consists of a composite attribute

    18. © Jae H. Park 2006 18 Associative Entities An entity type that associates the instances of one or more entity types and contains attributes that are peculiar to the relationship between those entity instances. the entity was initially specified as a relationship Usually relationship name (a verb) is converted to an entity name (a noun) How do we know when we have to convert a relationship to an entity? Many to many relationship Resulting entity type has independent meaning to users and can be identified with a single-attribute identifier Relationship includes attributes other than identifier

    19. © Jae H. Park 2006 19 Associative Entities

    20. © Jae H. Park 2006 20 Associative Entities

    21. © Jae H. Park 2006 21 Associative Entities

    22. © Jae H. Park 2006 22 Associative Entities

    23. © Jae H. Park 2006 23 Degree of Relationship Degree The number of entity types that participate in a relationship Unary relationship A relationship between the instances of a single entity type Binary relationship A relationship between the instances of two entity types Ternary relationship A simultaneous relationship among the instances of three entity types

    24. © Jae H. Park 2006 24 Unary Relationship

    25. © Jae H. Park 2006 25 Binary Relationship

    26. © Jae H. Park 2006 26 Ternary Relationship

    27. © Jae H. Park 2006 27 Ternary Relationship

    28. © Jae H. Park 2006 28 Attribute or Relationship? (multivalued attribute)

    29. © Jae H. Park 2006 29 Attribute or Relationship? (multivalued and composite attribute)

    30. © Jae H. Park 2006 30 Attribute or Relationship? (composite attribute)

    31. © Jae H. Park 2006 31 Attribute or Relationship?

    32. © Jae H. Park 2006 32 Cardinality Constraints Specifies the number of instances of one entity that can (or must) be associated with each instance of another entity Minimum Cardinality If minimum cardinality is zero, we say the entity type is an optional participant in the relationship In the previous slide, EMPLOYEE entity type is an optional participant in the “belongs” relationship Maximum Cardinality In the previous slide, maximum cardinality of EMPLOYEE is “many” (by crow’s feet) Minimum and maximum cardinality of DEPARTMENT in “belongs” relationship are both one. This is called a “mandatory one” cardinality. This means each employee must belong to one and only one department

    33. © Jae H. Park 2006 33 Time-dependent Data Modeling

    34. © Jae H. Park 2006 34 Time-dependent Data Modeling

    35. © Jae H. Park 2006 35 Multiple Relationships

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