我需要从3个不同的mysql表中获取一些数据,如下所示。
+-----------------------------------+
| Questions |
-------------------------------------
| id: int(8) |
| -- data we don't care about here |
| question: TEXT |
+-----------------------------------+
+--------------------------+
|Answers |
+--------------------------+
| id: int(8) |
| -- other data |
| answer: TEXT |
+--------------------------+
+-----------------------------------+
|Votes |
+-----------------------------------+
| id: int(8) |
| step: tinyint(1) |
| answerId: int(8) |
+-----------------------------------+
我需要获得每个问题的票数最多的答案,以及该答案所拥有的票数。
我能提出的最佳查询是:
SELECT question, answer, count(votes.id) votesCount FROM questions INNER JOIN answers ON questions.id = answers.questionId INNER JOIN votes ON answers.id = votes.answerId;
但是这是错误的,因为它总是返回带有第一个问题,一个随机答案和总票数的单个结果。
您的直接问题是您的查询缺少group by
子句。因此,它仅返回一行,并具有总票数和任意选择的问题和答案。
如果您正在运行MySQL 8.0,则可以使用聚合和窗口功能轻松完成此操作:
select question, answer, votescount
from (
select
q.question,
a.answer,
count(*) votescount,
rank() over(partition by q.id order by count(*) desc) rn
from questions q
inner join answers a on q.id = a.questionid
inner join votes on a.id = v.answerid
group by q.id, q.question, a.id, a.answer
) t
where rn = 1
在早期版本中,它有点复杂。一种选择是使用having
子句过滤汇总结果集,该子句返回每个组的最高计数:
select
q.question,
a.answer,
count(*) votescount,
rank() over(partition by q.id order by count(*) desc) rn
from questions q
inner join answers a on q.id = a.questionid
inner join votes on a.id = v.answerid
group by q.id, q.question, a.id, a.answer
having count(*) = (
select count(*)
from answers a1
inner join votes v1 a1.id = v1.answerid
where a1.questionid = q.id
group by a1.id
order by count(*) desc
limit 1
)