er - Entity-Re lationship Mode (C l h3) Yi Che n De of C...

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Entity-Relationship Model (Ch3) Yi Chen Arizona State University
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CSE 412/598 2 Outline Entities and attributes Relationships Structural constraints Cardinality ratio: M:N; N:1, 1:1 Participation constraint: total vs partial Roles of relationship Multi-way Relationships Weak entity sets Class hierarchies Design principles
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CSE 412/598 3 Purpose of ER Model The ER ( entity-relationship)  model is a widely-used conceptual  model that allows us to sketch the overall (logical) structure  design of a database Captures kinds of data and how they connect. Not  how data changes. ER model is independent of the DBMS to be chosen ER model is graphically represented: ER diagrams. Later: convert ER designs to relational DB designs.
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CSE 412/598 4 Entities and Relationships Entity  [instance] An  object  that  exists  and  is  distinguishable  from  other  objects  (e.g.,  students);  has  a  set  of  attributes    (e.g.,  student-id, name, major) Entity  Type/Set A set of similar entities (e.g., a set of students); similar to  a  class  in  OO.  Entity  sets  need  not  be  disjoint  (e.g.,  Relationship  [instance] An association among several entities (e.g., the instances of which students are taking what classes) Relationship  Type/Set A set of similar relationships (e.g., students taking  classes)
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CSE 412/598 5 Keys key   is a set of attributes for one entity  type such that no two entities of this type  agree on  all  the attributes of the key. We must designate a key for every entity  type.
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CSE 412/598 6 In an entity-relationship diagram: Entity type - rectangle Attribute - oval, with a line to the rectangle  representing its entity type. Key attributes - attribute names with underline ER Diagrams (I): Entity, Attributes Single-valued Multi-valued derived composite
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CSE 412/598 7 Example Entity type  Beers  has two attributes,  name  and  manf  (manufacturer). Each  Beers  entity has values for these two  attributes, e.g. (Bud, Anheuser-Busch) name  is the key of  Beers Beers name manf
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CSE 412/598 8 Example Courses dept number hours location   (dept, number) is a multi-attribute Key   Location is a composite attribute street city
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CSE 412/598 9 Relationships relationship  type connects two or more entity  types. The “value” of an entity type is the set of entities 
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This note was uploaded on 12/11/2009 for the course CSE 412 taught by Professor Davulcu,h during the Fall '08 term at ASU.

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er - Entity-Re lationship Mode (C l h3) Yi Che n De of C...

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