Chapter 1 and 2- ERModel (8-24)

Chapter 1 and 2- ERModel (8-24) - The Entity-Relationship...

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The Entity-Relationship Model Database Management Systems, R. Ramakrishnan and J. Gehrke 1 Entities ssn name lot Database Management Systems, R. Ramakrishnan and J. Gehrke 2 Employees ER Model Basics Entity : Real-world object distinguishable from other objects. An entity is described (in DB) using a set of attributes Database Management Systems, R. Ramakrishnan and J. Gehrke 3 Entity Set : A collection of similar entities. E.g., all employees All entities in an entity set have the same set of attributes Each entity set has a key Each attribute has a domain
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Relationships lot dname budget did name ssn since Database Management Systems, R. Ramakrishnan and J. Gehrke 4 Departments Employees Works_In ER Model Basics (Contd.) Relationship : Association among two or more entities. – E g , Attishoo works in Pharmacy department Database Management Systems, R. Ramakrishnan and J. Gehrke 5 E.g., Attishoo works in Pharmacy department. Relationship Set : Collection of similar relationships. An n-ary relationship set R relates n entity sets E1 ... En Each relationship in R involves entities e1 in E1, . .., en in En Relationships (Contd.) lot name ssn Database Management Systems, R. Ramakrishnan and J. Gehrke 6 Employees Reports_To subor- dinate super- visor Want to capture supervisor-subordinate relationship
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Relationships (Contd.) name Parts id Database Management Systems, R. Ramakrishnan and J. Gehrke 7 name Suppliers id Want to capture information that a Supplier s supplies Part p to Department d name Departments id Ternary Relationship name id Database Management Systems, R. Ramakrishnan and J. Gehrke 8 name Suppliers id name Departments id Parts Contract How are these different? name
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Chapter 1 and 2- ERModel (8-24) - The Entity-Relationship...

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