This is an interview question I came across
Given THREE tables
- GRADUATE – Graduate roll
- HONOURS – Honours Subjects
- GRADHONS – Bridge table to link Graduate with a particular Honours subject
The question – How should GRADHONS be defined in the datamodel so the datamodel enforces the constraint that a graduate may only have a single Honours subject?
The interviewer was not terribly enamoured of my suggestions
- Use just the Graduate Key as PK in the table GRADHONS
- Change the datamodel and reference Honours as part of the Graduate table.
My question is the same as the original interview question – How would you define a unique-1-1 relationship in a bridge table so it is enforced by the datamodel?
Both your suggestions sound reasonable. If you’d prefer to have a separate column in GRADHONS to reference GRADUATE, then make GRADHONS have a foreign key reference to GRADUATE, and make that column unique. Therefore there can only be a single row in GRADHONS for any GRADUATE.