PHPMYADMIN：EXISTS 子句中的访问字段

Question

我确定这只是一个打字错误，但是如何在 PHPMyAdmin 中正确编写以下查询？

SELECT DISTINCT `email_address` as tmp1 
FROM `already_customer_checks` 
WHERE `is_customer` = 0 
AND NOT EXISTS (
                    SELECT * 
                    FROM `already_customer_checks` 
                    WHERE `email_address` = tmp1 
                    AND `is_customer` = 1
                )

Error: #1054 - Unknown table field 'tmp1' in where clause

背景：我想获取 'is_customer' = 0 的所有电子邮件地址，并且 'is_customer' = 1 的 table 中不存在其他电子邮件地址。

非常感谢您！

Answer 1

要使用子查询执行此操作，您需要将别名 tmp1 放在 table 上，而不是列上。然后：

SELECT DISTINCT `email_address`
FROM `already_customer_checks` as tmp1 
WHERE `is_customer` = 0 
AND NOT EXISTS (
    SELECT * 
    FROM `already_customer_checks` 
    WHERE `email_address` = tmp1.`email_address`
    AND `is_customer` = 1
)

您也可以考虑@kmoser 提出的评论，如果不太清楚，可能更有效。根据 MySQL 文档：

A LEFT [OUTER] JOIN can be faster than an equivalent subquery because the server might be able to optimize it better—a fact that is not specific to MySQL Server alone.

但是，如果您使用@kmoser 提出的 SQL，您可能不希望将 email_address 列与 tmp1 别名。

PHPMYADMIN：EXISTS 子句中的访问字段

PHPMYADMIN: Access field in EXISTS clause

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