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我正在将三个表连接在一起，但是在其中一个表中没有任何信息，除非用户打开我们发送给他们的电子邮件

我目前正在使用这个 sql，但它似乎无法正常工作

SELECT email.senton, customer.*, count(tracker.viewed) as viewed 
FROM email_sent AS email, customer_detail AS customer, email_tracker AS tracker 
WHERE email.customer_id = customer.customer_id 
    AND customer.Email = tracker.emailaddress 
    AND customer.LeadOwnerId = '{$this->userid}'

这是因为 email_tracker 表中可能没有客户信息，除非客户打开了电子邮件

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2 回答 2

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试试这个：

SELECT email.senton, customer.*, COUNT(tracker.viewed) as viewed 
FROM email_sent email INNER JOIN customer_detail customer
    ON email.customer_id = customer.customer_id
LEFT JOIN email_tracker tracker 
    ON customer.Email = tracker.emailaddress
WHERE customer.LeadOwnerId = '{$this->userid}'

LEFT JOIN子句用于始终获取左侧的列和右侧的列（如果它们存在于连接中）...

根据您的评论编辑：
尝试更改COUNT(tracker.viewed)部分

CASE
    WHEN tracker.emailaddress IS NOT NULL THEN COUNT(tracker.viewed)
    ELSE 0
END
    as viewed

我不确定它是否有效，我无法测试它，但试一试

于 2011-09-22T07:10:25.677 回答
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您想要的行为是LEFT OUTER JOIN（也称为LEFT JOIN）。NULL如果该行在另一个表中不存在，它只是将值设置为。

SELECT email.senton, customer.*, COUNT(tracker.viewed) AS viewed
FROM email_sent AS email
    JOIN customer_detail AS customer USING(customer_id)
    LEFT OUTER JOIN email_tracker AS tracker ON customer.Email = tracker.emailaddress
WHERE customer.LeadOwnerId = '{$this->userid}'

JOINPS：使用s 而不是笛卡尔积（你用 s 做了什么）通常是一个更好的主意,。

于 2011-09-22T07:11:39.370 回答