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

title	aliases	tags
entity-relationship-diagrams	ERDs, ERD, entity relationship diagram, Entity Relationship Diagram	info201

cardinality identifying vs non identifying relationship

labels are important - but not always needed

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

0.1 subtypes

uses:

• model mutual exclusivity
• better for modelling not for implementation

0.2 parallel relationship

could model as separate relationships via staff subtypes not very common

also an example of recursive many-to-many relationships

0.3 recursive relationship

labels are critical usually 1:M can be 1:1 or M:M

0.4 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