Question

我有一个这样的数据库：
问题表：

Fragment fragment = getSupportFragmentManager().findFragmentById(R.id.fragment_container);
        if (fragment instanceof FormCallbacks) {
            FormCallbacks callbacks = (FormCallbacks) fragment;
            String userName = callbacks.getUserName();
            String address = callbacks.getAddress();
            String phone = callbacks.getPhone();
        }

答案表：

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

投票表：

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

我想计算每个问题的分数。一个问题的分数是对该问题及其所有答案的赞成与否票数之差。例如，QId为1的问题1（答案分别为2、3和7）的分数是-1，因为它具有3个正面投票和4个负面投票。
到目前为止，我只能计算给定问题的分数。代码是：

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

如何计算每个问题的分数，然后从得分最高的问题到得分最低的问题进行排序？

我想要的结果是4（分数为+2），9（分数为0），1（分数为-1）。

Answer 1

嗯。。。假设pubid与rid匹配，这基本上只是条件聚合：

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;

根据评论进行编辑：

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;

Answer 2

您可以结合使用LEFT JOIN和CASE来计算分数。例如：

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

如何按特定列对表进行汇总和分组

2 个答案: