Model relationships between tables
See this course’s project requirements
A four-table equipment-loan schema with stable keys, relationship definitions, integrity rules and checked edge cases.
Keep your working files in your own notebook or software. Save a private description, checklist, and work link in My Proof.
What to include
- Four entity definitions and base row counts.
- Stable primary keys and foreign keys.
- Relationship sketch and trace.
- Dependency/redundancy example.
- Integrity rules, three edge cases and limits.
Review and revise
Use these criteria to check your work and choose what to improve.
- Objects and events remain distinct.
- Keys reconstruct every required relationship.
- Facts are stored with their proper owner.
- Constraints and case decisions match the stated policy.
Want more background?
The idea in everyday life
Design a small fictional equipment-loan system before writing retrieval queries.
A foreign key stores the identity of a related row, not a second copy of all its facts.
Model relationships between tables — guided practice
Describe one-to-many links and locate foreign keys.
Work in design.md or on paper; no database installation is required. Fictional equipment library: types T1=Camera, replacement units 200; T2=Tripod, replacement units 40. Items I1 and I2 have type T1; I3 has type T2. Borrowers B1=River and B2=Sky are aliases. Loans: L1, I1, B1, returned=1; L2, I1, B2, returned=0; L3, I3, B1, returned=0. Rule: returned is 0 or 1; each item may have at most one active (returned=0) loan. Keep the base rows available throughout all six lessons.
1. Link types to items
Write Type 1 -> many Items; place type_id inside Item.
You should see: I1 and I2 both reference T1.
Many items can share one type.
2. Link items to loans
Write Item 1 -> many Loans over time; place item_id inside Loan.
You should see: L1 and L2 both reference I1.
An item can have several historical events.
3. Link borrowers to loans
Write Borrower 1 -> many Loans; place borrower_id inside Loan.
You should see: B1 has L1 and L3.
One borrower can participate in multiple events.
4. Explain the event bridge
Trace B2 -> L2 -> I1 -> T1.
You should see: Sky borrowed a Camera, and L2 is active.
Following keys answers a question without copying descriptions.
What to finish
A relationship sketch and one key-by-key trace.
Watch for this
Assuming one-to-many over history permits multiple simultaneous active loans without an additional rule.
How can I tell I understand it?
Complete the changed case, explain your result, and pass the three application checks.
Quick knowledge check
Answer all 3 questions. Get at least 2 right and the lesson completes automatically.
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Answer all 3 questions above and get at least 2 right. You can review the explanations and try again.
Go to the knowledge checkOptional course overview video · same video as Lesson 1
Database Design Course - Learn how to design and plan a database for beginners
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Source & access notes · checked 2026-09-12
Opened and inspected on 2026-09-12: Entities, keys and relationship multiplicities.
Access: Free public web page; no paid enrollment required.
Reuse: CC BY 4.0 except credited exceptions; independent tables and explanations.
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