Domain (Relational Model)

Context: FIT2094_MOC · the set of valid atomic values an attribute may hold · constrains type/format/range · enforced as domain integrity

Quick Revision

• 🎯 Objective: the set of valid atomic values an attribute may take ➔ fixes type, format, range.
• 📦 Core Components: $\text{dom}(\cdot )$ ➔ atomic values ➔ column integrity.
• ⚡ Key Constraint: values must be atomic (1NF); union-compatibility needs compatible domains.

📝 Core

1. The Domain

• Definition ➔ the set of valid atomic values an attribute can take.
• Specifies ➔ data type, format, valid range.
• Source ➔ every attribute of a relation draws from a domain.

2. Atomic Values

• Indivisible ➔ one value per attribute per tuple.
• Forbids ➔ a multivalued cell (dependants={Ali|12,Sara|9}) — 1NF.

3. Domains & Queries

• Column integrity ➔ all values of an attribute from one domain.
• Union-compatibility ➔ set ops need same arity + positionally compatible domains.

Key identities:

$\text{dom}(\text{custno})=\text{customer\_number},\text{dom}(\text{custname})=\text{name}$ $\text{dom}(\text{custadd})=\text{address},\text{dom}(\text{custcredlimit})=\text{credit\_limit}$

When It Flips: a domain is a logical constraint; the DBMS maps it to a concrete column type at physical design. The FIT1058 number sets are example numeric domains.

📊 Exam Execution Trace

Manual Execution Trace

Validating attribute values:

Step / State	Attribute	Value	In domain?
0 (Init)	—	—	—
1	Age	25	yes ($0$–$120$)
2	Age	”twenty”	no (not integer)
3	Age	200	no (out of range)

⚠️ Common Mistakes

• 💡 A value outside the domain is rejected ➔ an Age attribute holds integers in $0$–$120$, never the text “twenty” (column/domain integrity).

🧠 Active Recall

What is a domain, and how does it relate to data integrity?

• Hint: Valid-value set per attribute.

Answer

• Short answer: The set of valid atomic values (type/format/range); enforces column/domain integrity.
• Why: Same domain ➔ Age accepts $0$–$120$ integers, not “twenty”.

Why must domain values be atomic, and where do compatible domains matter?

• Hint: 1NF + union-compatibility.

Answer

• Short answer: Atomic = one value per cell (1NF); set operations need union-compatible relations (same arity, compatible domains).
• Why: Forbids multivalued cells ➔ dependants={Ali,Sara} breaks atomicity.