← ACARA Digital Technologies Years 9–10

Crow's foot notation for one and many

ERD notation where a single bar at a line's end means one and a three-pronged crow's foot means many. The crow's foot end is the foreign key side of the relationship.

Part of: ACARA Digital Technologies Years 9–10 — Entities, Relationships and Queries — Module 15, QCE Digital Solutions Unit 2 — Data-Driven Problems and Solution Requirements — Module 68, QCE Digital Solutions Unit 2 — Data and Programming Techniques — Module 95, QCE Digital Solutions Unit 2 — Prototype Data Solutions — Module 13, QCE Digital Solutions Unit 3 — Interactions Between Users, Data and Digital Systems — Module 106, WA Computer Science ATAR Year 11 — Data Management — Module 34

Understanding Crow's foot notation for one and many in ACARA Digital Technologies Years 9–10

What it does: RESULT is the many side, so it sits at the crow's foot end. Its studentId column is the foreign key that points back to the one side's primary key. A circle means zero is allowed, so the link is optional. A bar means at least one is needed, so the link is mandatory.

Common mistakes: One class has many students, so the STUDENT end needs a crow's foot (|{). Two bars (||) would mean one class has only one student. It should be CLASS ||--|{ STUDENT. The crow's foot is on the wrong end. One author writes many books, so the bar goes at AUTHOR and the crow's foot at BOOK: AUTHOR ||--o{ BOOK.

Class: _____________________  ·  Name: _____________________  ·  Date: _________

Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

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This module's key terms are also in a printable crossword (Modules 433–451) and word search (Modules 413–451).

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Learning
Duo
In crow's foot notation, what does a single bar at the end of a relationship line mean?
OneMany
Matches
STUDENT >|---|| RESULT (foreign key studentId placed in STUDENT)
↓
The crow's foot is drawn at STUDENT, which says many students per result. The foreign key also sits on the one side.
Applying
Meanings
Term
crow's foot
Meaning
ERD notation where a single bar at a line's end means one and a three-pronged crow's foot means many. The crow's foot end is the foreign key side of the relationship.
Practice
erDiagram
Testing
Gaps
Teacher --< Class
Check Answer
Spots
2 TEACHER ||--o{ CLASS : runs
3 CLASS ||--|| STUDENT : has

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Quick Questions

1. In crow's foot notation, what does a single bar at the end of a relationship line mean?
A. One ✓
B. Many
A single bar means exactly one. The three-pronged crow's foot is what means many.
2. In crow's foot notation, what does the three-pronged symbol at the end of a line mean?
A. One
B. Many ✓
The three prongs look like a bird's foot and mean many records can be linked at that end.
3. A line has a bar at the CLASS end and a crow's foot at the STUDENT end. How should you read it aloud?
A. One class has many students ✓
B. One student has many classes
Start at the bar end: one class. Then follow the line to the crow's foot end: many students.
4. Which end of a one-to-many line holds the foreign key?
A. The single bar (one) end
B. The crow's foot (many) end ✓
The many side stores the foreign key. For example, each order row holds the customer's ID. The one side holds the primary key.
5. A CUSTOMER has a single bar and an ORDER has a crow's foot. Which table contains the customerId foreign key?
A. ORDER ✓
B. CUSTOMER
ORDER is the crow's foot (many) end, so it is the table that stores the foreign key.

Match the Pairs

STUDENT ||---|< RESULT → One student can have many results, but each result belongs to exactly one student.
CLASS ||--o< STUDENT → One class can have zero or many students. The circle marks the many side as optional.
In a one-to-many relationship, the crow's foot end is the side that holds the foreign key. → results.studentId REFERENCES students(studentId)
Reading an ERD aloud: start at one entity, follow the line, and say 'one' for a bar and 'many' for a crow's foot. → One PRODUCT can appear in many ORDERS.
STUDENT >|---|| RESULT (foreign key studentId placed in STUDENT) → The crow's foot is drawn at STUDENT, which says many students per result. The foreign key also sits on the one side.
TEACHER ||--o| CLASS (a teacher has many classes) → A bar and circle at the CLASS end mean zero or one, not many. The line needs a crow's foot to show many classes.

Key Term

crow's foot
ERD notation where a single bar at a line's end means one and a three-pronged crow's foot means many. The crow's foot end is the foreign key side of the relationship.

Fill in the Blanks

1. # | means one, < means many Teacher |--< Class # One teacher has many classes
Hint: Type the bar symbol that marks the 'one' end of the line, next to Teacher.
2. # | means one, < means many Teacher |--< Class # One teacher has many classes
Hint: Type the crow's foot symbol that marks the 'many' end, next to Class.
3. House |--< Student # The foreign key is in the Student table
Hint: Type the name of the table at the crow's foot end, which holds the foreign key.
4. # | means one, < means many House |--< Student # One House has many Students
Hint: Type the word that describes what the crow's foot end means when you read the ERD aloud.
5. # | = mandatory, o = optional Teacher |--o< Class # A teacher may have zero classes
Hint: Type the optional marker that means a teacher might have no classes at all.

Spot the Error

Question 1
A erDiagram
B TEACHER ||--o{ CLASS : runs
C CLASS ||--|| STUDENT : has
Line C — One class has many students, so the STUDENT end needs a crow's foot (|{). Two bars (||) would mean one class has only one student. It should be CLASS ||--|{ STUDENT.
Question 2
A AUTHOR }o--|| BOOK : writes
B BOOK ||--o{ LOAN : has
C -- FK authorId is in BOOK
Line A — The crow's foot is on the wrong end. One author writes many books, so the bar goes at AUTHOR and the crow's foot at BOOK: AUTHOR ||--o{ BOOK.
Question 3
A ERD: OWNER ||--o{ PET : has
B Read: one PET has many OWNERS
C FK ownerId goes in PET
Line B — The bar is at OWNER and the crow's foot is at PET. So the diagram reads: one OWNER has many PETs, not the other way around.
Question 4
A erDiagram
B TEAM ||--o{ PLAYER : has
C PLAYER ||--o{ GOAL : scores
D %% FK teamId sits in TEAM
Line D — The foreign key goes in the table at the crow's foot (many) end. PLAYER is the many side, so teamId is the foreign key in PLAYER, not TEAM.
Question 5
A erDiagram
B FAN ||--o{ TICKET : buys
C %% A fan must have a ticket
Line C — The small circle (o) next to the crow's foot means optional (zero or many). A fan may have no tickets. A mandatory link would use a bar (|{) instead.

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