Free Keys, ER and normalization lessonsKeys, ER and normalization · Stage 1 · Modelling the real world · Step 1
Entities, relationships and cardinality
Turn a description of a business into entities, attributes and relationships, and read one-to-many and many-to-many correctly. About 13 minutes.
A hospital rule: a doctor may treat many patients over time, and a patient may be treated by many doctors. Where should the date of a treatment be stored?
- AOn the treatment relationship, since it belongs to a (doctor, patient) pair
- BOn the doctor
- COn the patient
Show the answer
On the treatment relationship, since it belongs to a (doctor, patient) pair. The date depends on both together, not on either alone.
4 questions on this idea
Short questions that apply the idea to new cases: trace it, work out a number, put steps in order. A wrong answer explains itself.
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