Using select from another select in MySQL - mysql

Question

I have the following query where I will have finally a 205 patient IDs to work with:
select
patient_id
FROM
visit
WHERE
month(visit.date_of_visit)=3
AND
year(visit.date_of_visit)=2018
AND
visit.visit_status='Active'
GROUP BY patient_id
I want to get all the 205 IDs and run them into other query to see how many diseases we have as cardio-vascular and then as respiratory disease.
My database structure is as the following:
What I want is to get for each patient id, what they are diagnosed at ONLY their first visit to the hospital (so here we will work with min(visit.date_of_visit))
The desired result for `diagnosis_name LIKE '%Cardio%':
E.g>:
Patients: 150 (Or something)
And the query is changed to get the respiratory info.
I tried the following for the Cardio diseases where I use select from select:
SELECT count(*)
FROM
(
select
min(visit.date_of_visit), visit_id, patient_id, count(*) as patientId
FROM
visit
WHERE
month(visit.date_of_visit)=3
AND
year(visit.date_of_visit)=2018
AND
visit.visit_status='Active'
GROUP BY patient_id
) as vid
LEFT JOIN
consultation ON consultation.visit_id=vid.visit_id
LEFT JOIN
diagnosis ON diagnosis.diagnosis_id=consultation.diagnosis_id
WHERE diagnosis.diagnosis_name LIKE '%Cardio%'
The result was: 5 which is a wrong number.
This can be done easily with PHP and MYSQL together but this will exhaust the server by repeating the same query for 205 times and increment a counter. So the desired result should be only done with MySQL.
Data example:
Visit Table
visit_id= 1; date_of_visit=2018-03-03; visit_reason=Active; ...;
patient_id=1234;
visit_id= 2; date_of_visit=2018-03-04; visit_reason=Active; ...;
patient_id=1239;
visit_id= 3; date_of_visit=2018-03-07; visit_reason=Active; ...;
patient_id=1234;
Consultation Table
consultation_id=1; ...; diagnosis_id=12; visit_id=1;...;
consultation_id=2; ...; diagnosis_id=12; visit_id=2;...;
Diagnosis Table
diagnosis_id=12; diagnosis_name: hypertension (cardio disease);
diagnosis_id=13; diagnosis_name: renal disease
By running the query to see patients who came to hospital and that they were diagnosed as having cardio disease in their initial first visit, the result should be in the example as 2 as you can see from the example where patient_id=1234 had 2 visits but I need to know what he had in his first one.

Answer 1

You can use window functions in MySQL 8+. But in older versions you need to calculate the value some other way.
The question for you is what you are counting:
SELECT COUNT(*) as num_diagnoses, COUNT(DISTINCT patient_id) as num_patients
FROM visit v JOIN
(SELECT patient_id,
MIN(v.date_of_visit) as min_dov
FROM visit v
WHERE v.date_of_visit >= '2018-03-01' AND
v.date_of_visit < '2018-04-01' AND
v.visit_status = 'Active'
) vf
ON v.patient_id = vf.patient_id AND v.date_of_visit = vf.min_dov JOIN
consultation c
ON c.visit_id = v.visit_id JOIN
diagnosis d
ON d.diagnosis_id = c.diagnosis_id
WHERE d.diagnosis_name LIKE '%Cardio%';
When working with dates, it is best to compare column values directly to dates, rather than dissecting them.

Answer 2

BRO, it works fine. Test it now on the live scenario.
SELECT count(*)
FROM
(
select
min(visit.date_of_visit) first_date, patient_id, count(*) as patientId
FROM
visit
WHERE
month(visit.date_of_visit)=3
AND
year(visit.date_of_visit)=2018
AND
visit.visit_status='Active'
GROUP BY patient_id
) as vid
INNER JOIN visit b ON
B.patient_id = vid.patient_id AND
B.date_of_visit = vid.first_date and
month(B.date_of_visit)=3 AND
year(B.date_of_visit)=2018 AND
B.visit_reason='Active'
INNER JOIN consultation ON
consultation.visit_id = B.visit_id
INNER JOIN diagnosis ON
diagnosis.diagnosis_id = consultation.diagnosis_id AND
diagnosis.diagnosis_name LIKE '%Cardio%'