join instances of same row - sql

Question

I have a table h containing data like this (OK, not really, it's just an example):
subj_id q1 q2 q3 q4 q5 q6 num
1 1 0 0 1 0 0 1
1 0 0 0 1 0 0 2
2 1 1 1 1 0 1 1
2 1 0 0 1 0 0 2
2 1 1 1 0 0 1 3
3 0 1 0 0 1 1 1
I would like to sum up the q's for each subj_id resulting in a output like this:
subj_id num1 num2 num3
1 2 1 null
2 5 2 4
3 3 null null
but instead I get the following:
subj_id num1 num2 num3
1 2 1 null
1 2 1 null
2 5 2 4
2 5 2 4
2 5 2 4
3 3 null null
where the summed rows are repeated as many times as the subj_id appears in the table.
My query (postgres) looks like this:
select h.subj_id, n1.sum as num1, n2.sum as num2, n3.sum as num3 from ((( h
left join (select subj_id, q1+q2+q3+q4+q5+q6 as sum from h where num=1) as n1 on h.subj_id=n1.subj_id)
left join (select subj_id, q1+q2+q3+q4+q5+q6 as sum from h where num=2) as n2 on h.subj_id=n2.subj_id)
left join (select subj_id, q1+q2+q3+q4+q5+q6 as sum from h where num=3) as n3 on h.subj_id=n3.subj_id) order by h.subj_id
Left join is obvious not the trick to use here, but what to do to skip the repeating rows?
Thanks in advance!

Answer 1

Your query could be easily modified to this:
with cte as (
select subj_id, q1 + q2 + q3 + q4 + q5 + q6 as q, num
from h
)
select
subj_id,
sum(case when num = 1 then q end) as num1,
sum(case when num = 2 then q end) as num2,
sum(case when num = 3 then q end) as num3
from cte
group by subj_id
order by subj_id
I think plan would be much better - no joins at all.
=> sql fiddle demo
Brief explanation why your query is not working and how you can improve it:
You receive more rows that you want because basically what your query does is selecting each row from table h and then join to it sum from table h with num = 1, 2, 3. You have 6 rows in your initial table and it's logical you'll have 6 rows in your result;
If you ever will make some query like this, I strongly suggest you to use aliases for tables in the inner queries. I'll help you to understand queries. In some cases it'll also help you to avoid incorrect results - see my answer in this topic - SQL IN query produces strange result.
-
select
h.subj_id, n1.sum as num1, n2.sum as num2, n3.sum as num3
from h
left join (
select h1.subj_id, h1.q1+h1.q2+h1.q3+h1.q4+h1.q5+h1.q6 as sum
from h as h1
where h1.num=1
) as n1 on h.subj_id=n1.subj_id
left join (
select h2.subj_id, h2.q1+h2.q2+h2.q3+h2.q4+h2.q5+h2.q6 as sum
from h as h2
where h2.num=2
) as n2 on h.subj_id=n2.subj_id
left join (
select h3.subj_id, h3.q1+h3.q2+h3.q3+h3.q4+h3.q5+h3.q6 as sum
from h as h3
where h3.num=3
) as n3 on h.subj_id=n3.subj_id
order by h.subj_id