Entity (Conceptual Modelling)

Context: FIT2094_MOC · a real-world object data is collected about · drawn as a Crow’s Foot box · strong (independent) vs weak (identity-dependent)

Quick Revision

• 🎯 Objective: a real-world object data is collected about ➔ a keyed Crow’s Foot box.
• 📦 Core Components: name / attributes / key column ➔ strong vs weak.
• ⚡ Key Constraint: weak entity needs a parent’s key in its own key + an identifying relationship.

📝 Core

1. The Entity

• Definition ➔ a real-world object/concept data is collected about (CUSTOMER, ORDER).
• Crow’s Foot box ➔ three partitions: name, attributes, key label.
• Needs a key ➔ uniquely identifies each instance.

2. Strong vs Weak

• Strong ➔ own unique key, exists independently (STUDENT/student_id).
• Weak ➔ can’t be identified alone; key includes a strong entity’s key (DEPENDENT re-uses emp_no).
• Crow’s Foot weak ➔ composite key with parent’s key + identifying relationship (solid line).

3. Multivalued Attribute

• No symbol ➔ Crow’s Foot has none, so a multivalued attribute (car_color) becomes its own weak entity.

Key identities:

$\text{PROFESSOR}(\underline{\text{prof\_id}},\text{name})(\text{strong})$ $\text{CLASS}(\underline{\text{prof\_id}},\underline{\text{class\_no}},\text{class\_day},\dots ),{\text{prof\_id}}^{\ast }(\text{weak})$

⚖️ Core Decision Matrix

Property	Strong	Weak
identity	self-contained	borrows parent’s key
key	own	composite incl. parent PK
relationship	any	identifying (solid)
example	EMPLOYEE	DEPENDENT

When It Flips: weak ⟹ identifying relationship — the two ideas always travel together. The identity test decides: if the entity's own attributes uniquely identify it, it is strong; if it must borrow a parent's key, it is weak.

📊 Exam Execution Trace

Manual Execution Trace

Deciding CLASS strong/weak:

Step / State	Check	Result
0 (Init)	—	—
1	is class_no unique alone?	no (restarts per prof)
2	needs prof_id?	yes
3	verdict	CLASS weak, PROFESSOR strong

⚠️ Common Mistakes

• 💡 class_no restarts per professor ➔ not unique alone, so CLASS’s key needs prof_id too — the identity test that makes it weak; a surrogate key would resolve it but is banned conceptually.

🧠 Active Recall

Distinguish a strong from a weak entity, and how each is shown in Crow's Foot.

• Hint: Own key vs borrowed key.

Answer

• Short answer: Strong = own unique key, independent; weak = key includes a parent’s key.
• Why: No double box ➔ Crow’s Foot marks weak by composite key + identifying (solid) relationship, unlike Chen’s double rectangle.

Given $(\text{prof\_id},\text{class\_no},\dots )$ with per-professor class numbering, is CLASS strong or weak?

• Hint: Uniqueness test.

Answer

• Short answer: CLASS is weak; PROFESSOR is strong.
• Why: Composite key ➔ class_no restarts per professor, so identity needs prof_id + class_no.