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IS 325 Notes for Wednesday September 4, 2013

IS 325 Notes for Wednesday September 4, 2013. Syllabus Change. I eliminated quizzes I increased the points allocated to homework assignments. Entity-Relationship Modeling/Diagrams. Entity-relationship (ER) modeling - conceptual database modeling technique

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IS 325 Notes for Wednesday September 4, 2013

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  1. IS 325 Notes for Wednesday September 4, 2013

  2. Syllabus Change • I eliminated quizzes • I increased the points allocated to homework assignments

  3. Entity-Relationship Modeling/Diagrams • Entity-relationship (ER) modeling - conceptual database modeling technique • Enables the structuring and organizing of the requirements collection process • Provides a way to graphically represent the requirements • ER diagram (ERD) - the result of ER modeling • Serves as a blueprint for the database

  4. ENTITIES • Entities - constructs that represent what the database keeps track of… something of specific interest • The basic building blocks of an ER diagram • Represent various real world notions, such as people, places, objects, events, items, and other concepts (think “nouns”) • Within an ERD each entity must have a different name and different semantic meaning

  5. ENTITIES Two entities

  6. ENTITIES • Entity instances (entity members) - occurrences of an entity • Entities themselves are depicted in the ER diagrams while entity instances are not • Entity instances are eventually recorded in the database that is created based on the ER diagram • Each entity instance becomes a row in the database

  7. ATTRIBUTES • Attribute - a characteristic of an entity • Represents the details that will be recorded for each entity instance • Within one entity, each attribute must have a different name and different semantic meaning • Unique Attribute/Set of Attributes – an attribute or set of attributes that have a value(s) is different for each entity instance • Every regular entity must have at least one unique attribute or a set of attributes (think George Foreman)

  8. ATTRIBUTES An entity with attributes

  9. RELATIONSHIPS • Relationship – meaningful or naturally occurring associations between entities • Within an ER diagram, each entity must be related to at least one other entity via a relationship

  10. RELATIONSHIPS • Cardinality constraints - depict how many instances of one entity can be associated with instances of another entity • Maximum cardinality • One (represented by a straight bar I (Roman Numeral 1)) • Many (represented by a crow’s foot symbol) • Minimum cardinality (participation) • Zero or Optional (represented by a circular symbol: 0 (Roman Numeral 0)) • Mandatory (represented by a straight bar I (roman Numeral 1))

  11. RELATIONSHIPS A relationship between two entities

  12. RELATIONSHIPS Four possible cardinality constraints

  13. RELATIONSHIPS Several possible versions of the relationship "ReportsTo"

  14. RELATIONSHIPS • Types of Relationships (maximum cardinality-wise) • One-to-one relationship (1:1) • One-to-many relationship (1:M) • Many-to-many relationship (M:N)

  15. RELATIONSHIPS Three types of relationships (maximum cardinality-wise)

  16. RELATIONSHIPS 1:M Relationship M:N Relationship 1:1 Relationship

  17. RELATIONSHIPS • Relationship instances - occurrences of a relationship • Occur when an instance of one entity is naturally or meaningfully associated to an instance of another entity via a relationship • Relationship themselves are depicted in the ER diagrams while relationship instances are not • Relationship instances are eventually recorded in the database that is created based on the ER diagram

  18. RELATIONSHIPS A relationship and its instances

  19. RELATIONSHIPS • Relationship attributes • In some cases M:N relationships can actually have attributes of their own

  20. RELATIONSHIPS A M:N relationship with an attribute

  21. RELATIONSHIPS A 1:M relationship with and without an attribute

  22. ER diagram example: ZAGI Retail Company Sales Department Database

  23. ATTRIBUTES • Composite attribute – attribute that is composed of several attributes (an amalgam or composite) • Not an additional attribute of an entity • Its purpose is to indicate a situation in which a collection of attributes has an additional meaning, besides the individual meanings of each attribute

  24. ATTRIBUTES An entity with a composite attribute

  25. ATTRIBUTES Another entity with a composite attribute

  26. ATTRIBUTES Composite attributes sharing components

  27. ATTRIBUTES • Composite unique attribute – attribute that is composed of several attributes and whose value is different for each entity instance

  28. ATTRIBUTES An entity with a unique composite attribute

  29. ATTRIBUTES • Multiple unique single attributes or minimal sets of attributes (candidate keys) - when an entity has more than one unique attribute each unique attribute is also called a candidate key

  30. ATTRIBUTES An entity with multiple unique attributes (candidate keys)

  31. ATTRIBUTES An entity with a candidate key and composite key (which does not satisfy the requirements of of a candidate key… I differ with the text authors)

  32. ATTRIBUTES • Multivalued attribute - attribute for which instances of an entity can have multiple values for the same attribute

  33. ATTRIBUTES A multivalued attribute

  34. ATTRIBUTES A scenario that does not use multivalued attributes

  35. ATTRIBUTES • Derived attribute - attribute whose values are calculated and not permanently stored in a database

  36. ATTRIBUTES A derived attribute example

  37. ATTRIBUTES Another derived attribute example

  38. ATTRIBUTES • Optional attribute - attribute that is allowed to not have a value

  39. ATTRIBUTES An optional attribute example

  40. ATTRIBUTES EXAMPLE: An entity with various types of attributes

  41. RELATIONSHIPS • Exact minimum and maximum cardinality in relationships • In some cases the exact minimum and/or maximum cardinality in relationships is known in advance • Exact minimum/and or maximum cardinalities can be depicted in ER diagrams

  42. RELATIONSHIPS A relationship with specific minimum and maximum cardinalities

  43. RELATIONSHIPS

  44. RELATIONSHIPS • Degree of a relationship - reflects how many entities are involved in the relationship • Binary relationship - relationship between two entities (degree 2 relationship) • Unary relationship (recursive relationship) - occurs when an entity is involved in a relationship with itself (degree 1 relationship)

  45. RELATIONSHIPS Unary relationship examples

  46. RELATIONSHIPS • Relationship roles - additional syntax that can be used in ER diagrams at the discretion of a data modeler to clarify the role of each entity in a relationship

  47. RELATIONSHIPS Unary relationships with role names

  48. RELATIONSHIPS A binary relationship with role names

  49. RELATIONSHIPS • Multiple relationships between same entities • Same entities in an ER diagram can be related via more than one relationship

  50. RELATIONSHIPS Multiple relationships between the same entities

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