Question

I need help with this design, a Diagnosis can be based on evidence or test result or both. I have separate evidence table (eg: cough, medicine can be prescribed with out any other test) and Test table (eg: blood test). I want the diagnosis table to address all scenarios (based on evidence only, based test result only, based on both evidence and test result. My table structure is Diagnosis_id, patient_id, evid_id, test_id,Dignosis_name. I want to refer evid_id and test_id as foreign key but having this would mean I must always provide both values, which wouldn’t be the case. Any advice?

Answer 1

A foreign key can be null.

You’d probably need a query like

select patients.name, diagnoses.id, evidence.text, tests.text
from patients inner join diagnoses on diagnoses.patient_id = patients.id
left join evidence on diagnoses.evid_id = evidence.id
left join tests on diagnoses.test_id = tests.id
where ....

Using left joins means that you’ll always get a result set, regardless of whether the diagnosis is linked to the evidence table or the tests table (or neither).