Crow's Foot ERD Notation

Crow's foot ERD notation cheat sheet: the four line-end symbols and the matching Mermaid syntax.

Syntax

design-and-specify

SYMBOL   MEANS           MERMAID
 ||      exactly one     ||
 o|      zero or one     |o  (left) / o| (right)
 |<      one or more     }|  (left) / |{ (right)
 o<      zero or more    }o  (left) / o{ (right)

Every relationship answers two questions, in BOTH directions:
  How many?          one / many
  Must there be one? mandatory / optional

Example

design-and-specify

SENTENCES FIRST, SYMBOLS SECOND
  "One member has zero or more loans."
  "Every loan belongs to exactly one member."

  MEMBER ||--o{ LOAN : borrows

  "One loan has one or more loan events."
  "Every loan event belongs to exactly one loan."

  LOAN ||--|{ LOAN_EVENT : records

MANY-TO-MANY HIDES AN ENTITY
  MEMBER }o--o{ TOOL          <- wrong: where does "wanted for" live?
  MEMBER ||--o{ RESERVATION : places
  TOOL   ||--o{ RESERVATION : "is wanted in"   <- right