SQL Delete with Join 和 Foreign Key 约束

Question

使用 10.0.31-MariaDB-1~jessie。

我有两个表:provider_contact 和 provider_contact_x_role 其中 provider_contact_x_role 表中的 contact_id 列有一个外键引用provider_contact 表中的 id 列。

当我运行以下查询时，结果是成功的:

DELETE cr, c
FROM provider_contact_x_role AS cr
INNER JOIN provider_contact AS c
ON cr.contact_id = c.id
WHERE c.is_test_contact = 0;

当我运行下一个查询时:

DELETE cr, c
FROM provider_contact_x_role AS cr
INNER JOIN provider_contact AS c
ON cr.contact_id = c.id
WHERE c.email_address <> 'suren@test.com';

结果是下面的错误:

Cannot delete or update a parent row: a foreign key constraint fails 
(`ins_db`.`provider_contact_x_role`, CONSTRAINT 
`FK_contact_id--provider_contact.id` FOREIGN KEY (`contact_id`) 
REFERENCES `provider_contact` (`id`))

请注意，这两个查询之间的唯一区别是 WHERE 条件。

问题:

1) SQL 中是否有任何内容可确保在执行这些查询时首先删除 provider_contact_x_role 中的行？

2) 知道这两个查询之间有什么区别吗(为什么第一个成功而第二个失败)？

Answer 1

摘自 MySQL 文档:

If you use a multiple-table DELETE statement involving InnoDB tables for which there are foreign key constraints, the MySQL optimizer might process tables in an order that differs from that of their parent/child relationship. In this case, the statement fails and rolls back. Instead, you should delete from a single table and rely on the ON DELETE capabilities that InnoDB provides to cause the other tables to be modified accordingly.

如果您使用 EXPLAIN，您会发现它更喜欢先按主表过滤(和删除)，因为它需要它来执行 JOIN。

如果您使用 INNER JOIN，您可以尝试 STRAIGHT_JOIN，它与 INNER JOIN 相同，只是左表始终是在右表之前阅读，并将表按照您想要实现的删除顺序排列。

除了级联，您还可以使用 SET FOREIGN_KEY_CHECKS=0; 在删除语句中禁用外键。