Multiple Full join is not working in PostgreSQL - database

Question

I have one select statement with multiple full joins in it.
select
aroll as aroll,
aname as aname,
astandard as astandard,
amarks as amarks,
adepartment_id as adepartment_id,
broll as broll,
bname as bname,
bstandard as bstandard,
bmarks as bmarks,
bdepartment_id as bdepartment_id,
croll as croll,
cname as cname,
cstandard as cstandard,
cmarks as cmarks,
cdepartment_id as cdepartment_id
from
(
(
(
select
roll as aroll,
name as aname,
standard as astandard,
marks as amarks,
department_id as adepartment_id
from student
where department_id = 1
and standard = 10) as firstA
full join
(select
roll as broll,
name as bname,
standard as bstandard,
marks as bmarks,
department_id as bdepartment_id
from student
where department_id = 2
and standard = 10) as secondB
on
firstA.astandard = secondB.bstandard
) first_second_combined
full join
(select
roll as croll,
name as cname,
standard as cstandard,
marks as cmarks,
department_id as cdepartment_id
from student
where department_id = 3
and standard = 10) thirdC
on
first_second_combined.astandard = thirdC.cstandard
and
first_second_combined.bstandard = thirdC.cstandard
)x;
It gives me error as below
Error: FULL JOIN is only supported with merge-joinable or hash-joinable join conditions

Answer 1

You are making it look very complicated. You could write your query like this:
select
firstA.roll as aroll,
firstA.name as aname,
firstA.standard as astandard,
firstA.marks as amarks,
firstA.department_id as adepartment_id,
secondB.roll as broll,
secondB.name as bname,
secondB.standard as bstandard,
secondB.marks as bmarks,
secondB.department_id as bdepartment_id,
thirdC.roll as croll,
thirdC.name as cname,
thirdC.standard as cstandard,
thirdC.marks as cmarks,
thirdC.department_id as cdepartment_id
from student firstA
full join student secondB ON firstA.standard = secondb.standard
full join student thirdC ON firstA.standard = thirdC.standard
It essentially means "Get the triplets of students sharing a common standard.".
Note that you will get a triplet (S, S, S) for each student S.
Also, for each triplet (A, B, C), you will also get `(A, C, B), (B, A, C), (B, C, A), (C, A, B), (C, B, A). If your intent is to get only triplets of different students, and also to avoid repetitions, you may want to add :
where firstA.name < secondB.name AND secondB.name < thirdC.name
Note also that using FULL JOIN here makes sense only if you have NULL values in the standard column. Otherwise you could aswell use INNER JOINs.
I hope this helps you get better knowledge of how SQL works.