Synthesis (Normalisation)
Context: FIT2094_MOC · combine the 3NF relations from multiple forms · merge relations representing the same thing · the final integration step
Quick Revision
- 🎯 Objective: merge 3NF relations from multiple forms representing the same subject ➔ final integration.
- 📦 Core Components: normalise each form independently ➔ merge same-PK relations (union of attributes).
- ⚡ Key Constraint: only for multiple forms; done last, after every form is at 3NF.
📝 Core
1. The Step
- Definition ➔ after each form is at 3NF, combine duplicate/overlapping relations that model the same thing.
- Result ➔ one relation holding all their attributes.
2. When & How
- Multiple forms ➔ each normalised independently (full UNF→3NF).
- Same subject ➔ different forms often produce relations for the same subject.
- Merge ➔ union of attributes → integrated, duplication-free schema.
3. Scope
- Same PK ⟹ same subject ➔ merge candidates.
- Single form ➔ no synthesis needed.
When It Flips: without synthesis, the same subject persists as two near-duplicate relations across forms — reintroducing cross-form redundancy and the anomalies normalisation removed. It reconciles bottom-up results, complementing top-down ER design.
📊 Exam Execution Trace
Manual Execution Trace
Merging DRONE:
| Step / State | Source | Attributes |
|---|---|---|
| 0 (Init) | — | — |
| 1 | Service DRONE | +drone_decom_date |
| 2 | Rental DRONE | drone_pur_date |
| 3 | merged | union of both |
⚠️ Common Mistakes
- 💡 Do it last ➔ merging before every form is at 3NF risks combining partially-normalised structure and reintroducing partial/transitive dependencies.
🧠 Active Recall
What is synthesis, when is it needed, and how do you decide which relations to merge?
- Hint: Same PK + subject.
Answer
- Short answer: Merge 3NF relations from multiple forms that model the same subject (same PK), keeping the union of attributes; only needed for multiple forms.
- Why: Overlap ➔ two
DRONE(drone_id, …)become one with all attributes.
Why is synthesis performed last, and what happens without it?
- Hint: After 3NF; cross-form redundancy.
Answer
- Short answer: Done after every form is at 3NF; without it the same subject persists as near-duplicate relations.
- Why: Reintroduces anomalies ➔ merging early risks partial/transitive dependencies.