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31 lines
835 B
Markdown
31 lines
835 B
Markdown
---
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title: "ER-Diagram"
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aliases:
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tags:
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- cosc203
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---
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Entity: a thing in the real world with *independent existence* (phsyical e.g., person or conceptual e.g., job)
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Entity type: A collection of entities
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Attribute: properties of entity
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Attibute types:
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- simple/composite
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- single/multi valued
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- stored/derived
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- key (simple or composite)
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Relationship: when one entity refers to another
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Relationship type: a set of relationships with the same entity types
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- relationships can have attributes
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- relationships have:
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- degree (bi/n-ary)
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- cardinality (1:1, 1:N N:M)
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- participation constraints (total (marked by double line) or partial)
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Weak entities
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- no key attributes
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- requires total participation constraint on identifying relationship to its identifying entity type
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- partial key used in combination with identifying entity
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