使用子选择值的 PostgreSQL 计算答案

【问题标题】：PostgreSQL Calculations Using Subselect Values使用子选择值的 PostgreSQL 计算
【发布时间】：2018-10-07 05:18:21
【问题描述】：

使用 PostgreSQL 9.3，我试图根据我们的电子邮件表中的子选择结果计算一些值。 以下是我想要做的，这当然会给出一个 SQL 错误，因为您无法在 SELECT 中引用别名值：

SELECT 
    email.key,
    email.col1,
    email.col2,
    (SELECT count(sent.subscriber_key) FROM sent WHERE sent.email_id = email.key) AS sent_count,
    (SELECT count(bounce.subscriber_key) FROM bounce WHERE bounce.email_id = email.key) AS bounce_count,
    ((bounce_count / sent_count) * 100) as bounce_pct
FROM
    email
ORDER BY 
    email.key

我已经尝试将子选择逻辑移到 WITH 子句中：

WITH
    sent_tmp AS (SELECT count(sent.subscriber_key) as count FROM sent INNER JOIN email ON sent.send_id = email.key),
    bounce_tmp AS (SELECT count(bounce.subscriber_key) as count FROM bounce INNER JOIN email ON bounce.send_id = email.key)
SELECT 
    email.key,
    email.col1,
    email.col2,
    email.sent_date,
    sent_tmp.count,
    bounce_tmp.count,
    ((bounce_tmp.count / sent_tmp.count) * 100) as bounce_pct
FROM
    email,
    sent_tmp,
    bounce_tmp
ORDER BY 
    email.key

...我已经尝试将它移到 FROM 子句中：

SELECT 
    email.key,
    email.col1,
    email.col2,
    sent_count,
    bounce_count,
    ((bounce_count / sent_count) * 100) as bounce_pct
FROM
    email,
    (SELECT count(sent.subscriber_key) FROM sent INNER JOIN email ON sent.send_id = email.key) AS sent_count,
    (SELECT count(bounce.subscriber_key) FROM bounce INNER JOIN email ON bounce.send_id = email.key) AS bounce_count
ORDER BY 
    email.key

...但无论哪种方式都会产生无效的已发送和退回号码。（每封电子邮件的数字都相同。）我可能不理解 PostgreSQL 查询语法的执行顺序，或者没有针对电子邮件表中的每一行结果评估子选择。

如果有人能指出我正确的方向并让我知道有什么可能，我们将不胜感激！将子选择逻辑移动到 WITH 或 FROM 时，我是否遗漏了一些关键语法？我需要使用命名变量吗？函数？

我意识到可以添加另一个子选择来重新计算已发送和退回表中的subscriber_key 列，但为了提高效率，我试图避免重新计算。（就此而言，还有可读性。）

【问题讨论】：

标签： postgresql join subquery alias postgresql-9.3

【解决方案1】：

简单的解决方案是在外部查询中计算值：

SELECT key, col1, col2, sent_count, bounce_count,
       100.0 * bounce_count / sent_count AS bounce_pct
FROM (SELECT key, col1, col2,
            (SELECT count(sent.subscriber_key)
             FROM sent
             WHERE sent.email_id = email.key
            ) AS sent_count,
            (SELECT count(bounce.subscriber_key)
             FROM bounce
             WHERE bounce.email_id = email.key
            ) AS bounce_count
      FROM email
     ) q
ORDER BY key;

【讨论】：