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quartz/content/notes/Entity Relationship Diagrams.md
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aliases
ERD, ERDs

Entity Relationship Diagrams

!Pasted image 20220328165256.png

cardinality identifying vs non identifying relationship

labels are important - but not always needed

associative entity => changes many to many relationship with additional relationship

1 subtypes

!Pasted image 20220328165640.png !Pasted image 20220328165656.png

uses:

  • model mutual exclusivity
  • better for modelling not for implementation

2 parallel relationship

!Pasted image 20220328165851.png

could model as separate relationships via staff subtypes not very common

!Pasted image 20220328165944.png also an example of recursive many-to-many relationships

3 recursive relationship

labels are critical usually 1:M can be 1:1 or M:M !Pasted image 20220328200337.png

4 dealing with data history

!Pasted image 20220328200434.png

could be many to many relationships:!Pasted image 20220328200517.png

so associative relationship: !Pasted image 20220328200536.png

what do we require:

  • for the current point in time
  • an histroical record how ⇒ must be selecetive to not use up to much space