我该如何复制列

Question

我需要返回此查询的笛卡尔积和一个计算计数列平均值的新列。

select site_topics.description as topic,
site_subscriptions.site_user_id as user1,
count(*) as count1 
from (
    (site_topics_to_subscriptions
    inner join site_subscriptions 
    on site_subscriptions.id = site_subscription_id)
    inner join site_topics
    on site_topics.id = site_topics_to_subscriptions.topic_id
    )
group by user1,topic
order by user1, count1 desc;

当前结果：

-----------------------------------
topic       |user1      |count1
-----------------------------------
Gaming      |1          |3
Photography |1          |3
Art         |1          |1
Gaming      |2          |2
Photography |2          |1
Art         |2          |1

所需的输出

-----------------------------------------------------
topic       |user1  |count1 |user2  |count2 |Average
-----------------------------------------------------
Gaming      |1      |3      |2      |2      |0.66
Photography |1      |3      |2      |1      |0.33
Art         |1      |1      |2      |0      |0
Gaming      |2      |2      |1      |3      |0.66
Photography |2      |1      |1      |3      |0.33

＆＃39;平均＆＃39;一个列，用该行的较大数量来计算较小的计数。同样的行也不会相互配对。即，不存在具有user1 = 1和user2 = 1

的行

Answer 1

一般结构应该是：

SELECT q1.user AS user1, q1.count AS count1, q2.user AS user2, q2.count AS count2, GREATEST(q1.count, q2.count)/LEAST(q1.count, q2.count) AS average
FROM (your query here) AS q1
JOIN (your query here) AS q2
ON q1.user < q2.user AND q1.topic = q2.topic

然后将获取原始表的查询替换为your query here。创建一个执行原始查询的视图可能会有所帮助，因此您不必在交叉产品中重复整个过程。