用GROUP BY計算COUNT的百分比

Question

這是兩個表和唯一與該問題有關的列。

郵寄M
m_id：三種郵件類型中的每一種的ID

郵件接收MR
u_id：收到郵件的人的ID m_id：FK

我已成功創建此查詢

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

它返回一個看起來像這樣的表

Mailing Type     Mailings Received
1                500
2                350
3                600

太棒了 真。 但是，我試圖返回第三列，該列告訴您按郵件類型接收的郵件百分比，並且進展不順利。 到目前為止，這是我在子查詢中嘗試過的內容：

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;

沒有骰子。

編輯： u_id通過COUNT和GROUP BY函數分為三行。 我不知道當返回行被分割時如何創建百分比列。

Answer 1

這應該工作：

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