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48 lines
1.1 KiB
Markdown
48 lines
1.1 KiB
Markdown
---
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title: "entity-relationship-diagrams"
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tags:
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- info201
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---
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cardinality
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identifying vs non identifying relationship
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labels are important - but not always needed
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associative entity => changes many to many relationship with additional relationship
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## 1 subtypes
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uses:
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- model mutual exclusivity
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- better for modelling not for implementation
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## 2 parallel relationship
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could model as separate relationships via staff subtypes
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not very common
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also an example of recursive many-to-many relationships
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## 3 recursive relationship
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labels are critical
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usually 1:M can be 1:1 or M:M
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## 4 dealing with data history
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could be many to many relationships:
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so associative relationship: 
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what do we require:
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- for the current point in time
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- an histroical record how ⇒ must be selecetive to not use up to much space |