Question

我正在尝试对表'apst_mailings'运行查询，存储我们发送给订阅者的每个简报的内容。每当我们尝试将电子邮件发送给个人时，我们在apst_mailings_accuses中插入一行，报告发送的时间和状态以及新邮件的ID。我想列出新闻通讯并计算每个通讯的成功发送总数。

    SELECT m.id AS code_mailing, 
    COUNT(a.adh_code) AS num, COUNT(b.adh_code) AS succes
    FROM apst_mailings AS m
        LEFT OUTER JOIN apst_mailings_accuses AS a
            ON a.id_mailing = m.id
        LEFT OUTER JOIN apst_mailings_accuses AS b
            ON b.id_mailing = m.id
            AND b.etat = 'succes'
    GROUP BY m.id

它只是永远挂起服务器。我已尝试在分离的查询中进行每个连接，并且它没有问题：

// Counts the email sent per mailing
    SELECT m.id AS code_mailing, 
    COUNT(a.adh_code) AS num
    FROM apst_mailings AS m
        LEFT OUTER JOIN apst_mailings_accuses AS a
            ON a.id_mailing = m.id
    GROUP BY m.id

    SELECT m.id AS code_mailing, 
    COUNT(b.adh_code) AS succes
    FROM apst_mailings AS m
        LEFT OUTER JOIN apst_mailings_accuses AS b
            ON b.id_mailing = m.id
            AND b.etat = 'succes'
    GROUP BY m.id

我可以分割我的查询，但它不起作用的原因真的困扰我。有人可以解释一下吗？

谢谢！

Answer 1

通过使用单个联接并使用SUM执行条件计数，您可以更简单的方式获得所需内容。

SELECT
   m.id AS code_mailing, 
   COUNT(a.adh_code) AS num,
   SUM(a.etat = 'succes') AS succes
FROM apst_mailings AS m
LEFT OUTER JOIN apst_mailings_accuses AS a
ON a.id_mailing = m.id
GROUP BY m.id

但是您的查询不起作用的原因是因为您正在加入 ALL 子查询 a 中的行 ALL 匹配子查询中的行 b 在一个巨大的交叉连接中。这可能会生成一个巨大的临时结果集，这可能是为什么查询需要永远终止的原因。即使它确实终止了，你的计数也会完全消失 - 它们将成为两个计数的产物。

要解决此问题，请先执行GROUP BY。然后JOIN将结果发送到主表。

SELECT
   m.id AS code_mailing, 
   IFNULL(a.num, 0) AS num,
   IFNULL(b.succes, 0) AS succes
FROM apst_mailings AS m
LEFT OUTER JOIN (
   SELECT id_mailing, COUNT(adh_code) AS num
   FROM apst_mailings_accuses 
   GROUP BY id_mailing
) a
ON a.id_mailing = m.id
LEFT OUTER JOIN (
    SELECT id_mailing, COUNT(adh_code) AS succes
    FROM apst_mailings_accuses
    WHERE etat = 'succes'
    GROUP BY id_mailing
) b
ON b.id_mailing = m.id

2在同一个表上的LEFT OUTER JOIN冻结服务器

1 个答案: