MySQL - Counting the number of patients

Question

I have a patient table and a reference table as below

patient table

id         name
1          Doni
2          Doni
3          Doni
4          Tejo
5          Bambang

reference table

patient_id    refer_to_patient_id
1             2
2             3
3             null

refer_to_patient_id null means patient name Doni is referred to another Hospital but the Hospital it's referred to has not created a record.

And I need to count how many patient they have. Example above should return 3 patients as record id 1,2,3 is the same patient. Patient name could be different even for the same patient since they are inputted from different hospitals.

PS: patient table is actually not patient entity, it's a medical record (EMR) and a hospital can refer a patient (or an EMR) to another hospital and it is counted as a single patient.

Answer 1

While initially it seems like you need to resolve the full tree, I think that the question you're asking is as simple as

select 
  count(distinct pat.id) patients
from 
  patient pat 
  left outer join reference ref on pat.id = ref.patient_id
where 
  ref.patient_id is null;

That is, an id should only be counted when it does not have a map to another id . Put another way, any id value that does not have a mapped value is the end of the hierarchy.

Answer 2

MySQL has a DISTINCT keyword.

SELECT count(DISTINCT name)
FROM patient;