Question

以下是两个表格和唯一对此问题重要的列。

邮寄M
m_id：三种邮件类型

中的每一种的ID

邮寄收到的MR
u_id：收到邮件的人的ID m_id：FK

我已成功创建此查询

SELECT 
    m.m_id AS 'Mailing Type',
    COUNT(mr.u_id) AS 'Mailings Received'
FROM mailing m 
Left join
mailing_received mr
ON mr.m_id = m.m_id
GROUP BY m.m_id

返回一个看起来像这样的表

Mailing Type     Mailings Received
1                500
2                350
3                600

哪个太棒了。真。但我试图返回第三列，告诉邮件类型收到的邮件的百分比，并不是那么顺利。以下是我到目前为止在子查询中尝试过的内容：

SELECT
m.m_id,
u_id * 100/ (SELECT COUNT(*) FROM mailing_received) AS "PERC REC"
FROM mailing m
LEFT JOIN
mailing_received mr
ON
m.m_id = mr.m_id
GROUP BY m.m_id;

没有骰子。

编辑：u_id通过COUNT和GROUP BY函数分为三行。当返回行被分割时，我不知道如何创建一列百分比。

Answer 1

这应该有效：

SELECT 
    m.m_id AS "Mailing Type",
    COUNT(mr.u_id) AS "Mailings Received",
    COUNT(mr.u_id) / (SELECT COUNT(*) FROM mailing_received) AS "PERC REC"
FROM mailing m 
Left join mailing_received mr
       ON mr.m_id = m.m_id
GROUP BY m.m_id

使用GROUP BY计算COUNT的百分比

1 个答案: