How to sum and group a table by a specific column

问题

I have a database like this:
Question table:

QId | Title
----------
1   | SR
4   | TR
9   | AA

Answer table:

RId | QId
----------
2   | 1
3   | 1
5   | 4
6   | 4
7   | 1
8   | 4

Vote table:

PubId | Type
-------------
2     | P
3     | N
3     | N
1     | P
1     | N
5     | P
4     | P
2     | N
2     | P

I want to calculate the score of every question. The score of a question is the difference between positive and negative votes on that question and all of its answers. For example the score of question 1 (with answers as 2, 3 and 7) with QId of 1 is -1 because it hase 3 positive votes and 4 negative votes.
So far I can only calculate the score of a given question. The code is:

SELECT P+N FROM (
SELECT sum (case WHEN Type='P' then +1 else 0 end) as P,
sum (case WHEN Type='N' then -1 else 0 end) as N
from ( 
SELECT v.Type from Vote v where v.PubId in (
SELECT r.RId FROM Answer r WHERE r.QId=4
UNION
select q.QId from Question q where q.QId=4)
)
)

How can I calculate the score of every question and then sort them from the question with the highest score to the question with lowest score?

The result that I want is 4 (with the score of +2), 9 (with the score of 0), 1 (with the score of -1).

回答1:

You can use LEFT JOINs combined with CASE to compute the scores. For example:

select
  *
from (
  select
    q.qid,
    sum(case when v.type = 'P' then 1 
             when v.type = 'N' then -1 else 0 end
       ) as score
  from question q
  left join answer a on a.qid = q.qid
  left join vote v on v.pubid = a.rid
  group by q.qid
) x
order by score desc

回答2:

Hmmm . . . assuming that pubid matches rid, this is basically just conditional aggregation:

select a.qid,
       sum(type = 'P') - sum(type = 'N') as score
from answer a join
     vote v
     on v.pubid = r.rid
group by a.qid;

EDIT based on coment:

select q.qid,
       coalesce(sum(type = 'P'), 0) - coalesce(sum(type = 'N'), 0) as score
from question q left join
     answer a
     on q.qid = a.quid left join
     vote v
     on v.pubid = r.rid
group by q.qid
order by score desc;

来源：https://stackoverflow.com/questions/61988086/how-to-sum-and-group-a-table-by-a-specific-column

标签

mysql

sql

sorting

sum