Attribute (Conceptual Modelling)

Context: FIT2094_MOC · a property of an entity · key vs non-key · types: simple/composite, single/multi-valued, derived

Quick Revision

• 🎯 Objective: a property describing an entity ➔ key attributes identify, non-key describe.
• 📦 Core Components: simple/composite ➔ single/multi-valued ➔ derived.
• ⚡ Key Constraint: a multivalued attribute has no Crow’s Foot symbol → becomes a separate (weak) entity.

📝 Core

1. The Attribute

• Definition ➔ a property of an entity.
• Key ➔ uniquely identifies instances (custno); flagged Key in the first column.
• Non-key ➔ describes but doesn’t identify (custname, custaddress).

2. Three Classification Axes

• Simple vs composite ➔ indivisible (age) vs subdividable (address→street/city/postcode).
• Single vs multi-valued ➔ one value (tax_file_number) vs several (degrees).
• Derived ➔ computed from others (age from date_of_birth; order total from lines).

3. Multivalued → Weak Entity

• No symbol ➔ Crow’s Foot lacks a multivalued marker (unlike Chen).
• Resolution ➔ store as a separate (usually weak) entity (CAR_COLOR).

Key identities:

$\text{CUSTOMER}(\underline{\text{custno}},\text{custname},\text{street},\text{city},\text{postcode})$ $\text{ORDER}(\underline{\text{orderno}},\text{orderdate})\text{[total = }\Sigma \text{ line items, derived]}$

⚖️ Core Decision Matrix

Axis	Types	Logical effect
structure	simple / composite	composite splits into parts
multiplicity	single / multi-valued	multi → new entity
origin	stored / derived	derived carried with sources
role	key / non-key	key → PK

When It Flips: classification is client-driven — the same field (phone number) may be simple or composite, single- or multi-valued, depending on requirements. Names share an entity prefix (cust_…), underscores not spaces.

📊 Exam Execution Trace

Manual Execution Trace

Classifying attributes:

Step / State	Attribute	Axes
0 (Init)	—	—
1	age	simple, single, derived
2	address	composite, single
3	degrees	simple, multi-valued

⚠️ Common Mistakes

• 💡 Carry a derived attribute alongside its sources ➔ during design always include every client-required attribute; store-vs-compute is decided later with sample data, not at conceptual stage.

🧠 Active Recall

Classify attributes along the three axes with an example of each.

• Hint: Three independent axes.

Answer

• Short answer: Simple/composite (age/address); single/multi-valued (tfn/degrees); derived (age from DOB).
• Why: Key cuts across ➔ a Key attribute identifies; non-key describes.

How is a multivalued attribute represented in Crow's Foot, and why?

• Hint: No symbol.

Answer

• Short answer: As a separate (weak) entity — car_color → CAR_COLOR.
• Why: Relations are single-valued ➔ each value gets its own row linked to the parent.