Each patient is cared for by one doctor. Each doctor cares for
A test must be done on some patient. Each test is done on
only one patient. A patient can have many tests, including
repetitions of the same test type.
The doctor who ordered a given test can be identified since the
care and log relationships are both one-to-many.
Use a ternary relationship to allow a patient to be cared for by
many doctors, while still allowing identification of the doctor who
ordered a given test.
The arrow notation is inadequate for representing the mapping
cardinality constraints properly. Instead, use two sets of written
constraints for each entity set, one for each of the other two
related entity sets.
This design cannot represent a doctor-patient relationship
without some test being involved.
Use a binary relationship for care, and a separate ternary
relationship for log. This is not redundant if a test can be ordered
by a doctor who is not considered to be caring for a patient.
This allows a doctor-patient relationship to be represented even
if no tests are done. But in most cases the two relationships will
have considerable redundancy.
Use aggregation so that the doctor-patient relationship can be
established in the absence of any tests. Then each test is seen as
related to a doctor-patient pair. This removes the redundancy of the