quartz/content/notes/ER-Diagram.md

title	aliases	tags
ER-Diagram		cosc203

Entity: a thing in the real world with independent existence (phsyical e.g., person or conceptual e.g., job) Entity type: A collection of entities

Attribute: properties of entity Attibute types:

• simple/composite
• single/multi valued
• stored/derived
• key (simple or composite)

Relationship: when one entity refers to another Relationship type: a set of relationships with the same entity types

• relationships can have attributes
• relationships have:
  • degree (bi/n-ary)
  • cardinality (1:1, 1:N N:M)
  • participation constraints (total (marked by double line) or partial)

Weak entities

• no key attributes
• requires total participation constraint on identifying relationship to its identifying entity type
• partial key used in combination with identifying entity