How to select all joined rows from 2 tables including null - mysql

Question

I currently have 2 tables, vaccination and vaccination_type which can be joined using vaccine_id attribute.
vaccination is basically the joining many-to-many table between vaccination_type and patient. It has 3 attributes: vaccine_id, registration_no and date. Vaccination_type has vaccine_id, vaccine_name.
I want to join these 2 tables and get these entries:
vaccine_id vaccine_name registration_no date
1 influenza 1111 2015-01-15
2 hepatitis B null null
3 polio 1111 2015-01-15
4 hepatitis A 1112 2015-01-15
This means that even the patient has not done hepatitis B vaccination, I still want the entry to be null. But I only want the one to specific registration_no, in this case 1111, in other words I also want the hepatitis A is written as null, as it is not done by 1111. So my expected result if I am dealing with 1111 will be:
vaccine_id vaccine_name registration_no date
1 influenza 1111 2015-01-15
2 hepatitis B null null
3 polio 1111 2015-01-15
4 hepatitis A null null
I have tried using all of the join, but cannot find the one that works perfectly. Any suggestion will be appreciated, thanks!

Answer 1

If you want the information for one registration, then that should be your base table. Then you want to get all vaccination types for that registration. Finally join onto the vaccination table to get the date it was administered, or NULL if that person never got it:
SELECT
registrations.registration_no,
vaccination_type.vaccine_id,
vaccination_type.name,
vaccination.date
FROM
registrations
CROSS JOIN vaccination_type
LEFT JOIN vaccination ON registrations.registration_no = vaccination.registration_no AND vaccination_type.vaccine_id = vaccination.vaccine_id
WHERE
registrations.registration_no = 1111
Fiddle: http://sqlfiddle.com/#!2/e8b2d/5

Answer 2

Use LEFT JOIN it join both tables.
Try this:
SELECT VT.vaccine_id, VT.vaccine_name, V.registration_no
FROM Vaccination_type VT
LEFT JOIN vaccination V ON VT.vaccine_id = V.vaccine_id;

Answer 3

As I now understand the question, you need a listing of all vaccinations against which you will need to see all patients (or a particular patient) to see which vaccinations a patient has and has not taken. Yes, that is possible, but it requires all three tables. The Vaccination_Type table has the list of all vaccinations, the Patients table has the list of all patients. The Vaccinations table has the list of vaccinations that patients have taken.
The first step is to create a Cartesian product of all vaccinations with all patients. This will give you a list of all vaccinations and for each vaccination a list of all patients.
select vt.vaccine_id, p.registration_no
from Vaccination_Type vt
cross join Patients p;
Then left join with the existing cross table.
select vt.vaccine_id, p.registration_no, v.vaccination_date
from Vaccination_Type vt
cross join Patients p
left join Vaccinations v
on v.vaccine_id = vt.vaccine_id
and v.registration_no = p.registration_no;
This gives you all vaccinations with all patients. If the patient has received the vaccination, it will be followed by the date of that vaccination. If not, the date field will be NULL.
If this is something you need periodically, like for reporting, you can create a view of the above query and just select from the view.
If you are only interested in seeing this listing in regards to a particular patient, you can select from the view:
select vaccine_id, registration_no, vaccination_date
from All_Vaccinations
where registration_no = 1111;
However, if a particular patient's vaccination history is mostly what you want, you can obtain that with just two tables with a little better performance:
select vt.vaccine_id, v.vaccination_date
from Vaccination_Type vt
left join Vaccinations v
on v.vaccine_id = vt.vaccine_id
and v.registration_no = 1111;
Notice that registration_no is left out of the select list as we are no longer using the Patients table. We could have used the registration_no field from the Vaccinations table but then if the patient is missing a vaccination, not only the date field but also the registration_no field of the result set would contain NULL. There is no reason to expose a field that A) you already know the value of and B) will have the proper value in some rows and NULL in others. That could lead to some confusion. When you are looking at the data for more than one patient at a time, you must use the three-table join as the registration_no would be required to make sense of the output.