Migrating data from one database to another using LEFT JOIN

Question

Let me start by saying I am relatively new to MySQL. I am trying to migrate data from one database to another. Let us call one database DB1 and the other DB2.

DB2 has tables the following tables.

Patient: id, person_id and start_regimen_id

Regimen: id, code

Visit: id, patient_id, regimen_id,next_appointment_date

DB1 has the following tables:

Patient: id,medical_record_number,current_regimen, start_regimen nextappointment

Now:

regimen_id data should be inserted to current_regimen

start_regimen_id data should be inserted to start_regimen

next_appointment_date should be inserted to nextappointment

This is what I have now:

                 SELECT
                    p.person_id AS medical_record_number,
                    r.code AS start_regimen,
                    ??? AS current_regimen,
                    DATE_FORMAT(v.next_appointment_date, '%Y-%m-%d') AS 
                    nextappointment,
                FROM patient p
                LEFT JOIN regimen r ON r.id = p.start_regimen_id
                LEFT JOIN (
                    SELECT patient_id, MAX(next_appointment_date) as next_appointment_date 
                    FROM visit 
                    WHERE next_appointment_date IS NOT NULL 
                    GROUP BY patient_id
                ) v ON v.patient_id = p.id

I have remained to migrate regimen_id (visit) on DB2 to current_regimen (patient) on DB1. I don't know how to use two LEFT JOIN to get data from two tables for one table.

Any assistance will be greatly appreciated because I am really stuck.

Answer 1

Seems like we would want to include regimen_id in the GROUP BY from the visit table, and then match that to id from the regimen table. Given the outer join, it appears that patient may not have any regimen associated, so I would include matching of NULL values of regimen_id .

    LEFT
    JOIN ( SELECT vv.patient_id
                , vv.regimen_id
                , MAX(vv.next_appointment_date) AS next_appointment_date 
             FROM visit vv
            WHERE vv.next_appointment_date IS NOT NULL 
            GROUP
               BY vv.patient_id
                , vv.regimen_id
         ) v
      ON v.patient_id = p.id
     AND v.regimen_id <=> r.id

But that's just a guess. Without a specification (preferably illustrated by example data and and expected output) we're just guessing.

Note:

foo <=> bar

is a NULL-safe comparison, a shorthand equivalent to

( foo = bar OR ( foo IS NULL AND bar IS NULL ) )