quartz/content/notes/entity-relationship-diagrams.md

title	draft	aliases
Entity Relationship Diagrams	true	ERD, ERDs

Entity Relationship Diagrams

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

uses:

• model mutual exclusivity
• better for modelling not for implementation

2 parallel relationship

could model as separate relationships via staff subtypes not very common

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

4 dealing with data history

could be many to many relationships:

so associative relationship:

what do we require:

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