ER model relationship

Description:

• Entities can participate in relationships with other entities.
  • For example: CUSTOMER buys ITEM

Binary relationship:

• Degree 2 meaning 2 entity instances are related in a relationship
• Many-to-many relationship
• One-to-many relationship
• One-to-one relationship

Ternary relationship:

• Degree 3 meaning 3 entity instances are related in a relationship
  • Example: 2 parents to have 1 children

Cardinality:

• Specifies the requirement of how many entity instance can be from or to in a relationship
  • Min-max cardinalities:
    • Example: a department can exist with minimum one employee
    • Typically min is 0 or 1
    • In Mermaid entity relationship diagram, use relationship syntax

Recursive relationship

• It is possible for an entity to have a relationship to itself—this is called a recursive relationship (also known as a unary relationship)

• 
  erDiagram
  CUSTOMER ||--o{ CUSTOMER: "refers to"
  CUSTOMER {
  string name
  string custNumber
  string sector
  }