Candidate Key:Doctor,ShiftID
Alternate Key: DoctoeDutyID
In the oreceding table, DoctorID and ShiftID are individually unique in every row.
Therefore, the columns, DoctorID and ShiftID, are candidate keys for the primary key.
However, the ShiftID column may contain duplicate values as two doctors may be on
the same shift. Therefore,DoctorID should be chasen as the primary key and ShiftID
as the alternate key.
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