如何暂时禁用MySQL中的外键约束?

是否可以暂时禁用MySQL中的约束?

我有两个Django模型，每个都有一个指向另一个的外键。删除一个模型的实例将返回一个错误，因为外键约束:

cursor.execute("DELETE FROM myapp_item WHERE n = %s", n)
transaction.commit_unless_managed()  #a foreign key constraint fails here

cursor.execute("DELETE FROM myapp_style WHERE n = %s", n)
transaction.commit_unless_managed()

是否有可能暂时禁用约束并删除?

当前回答

与其禁用约束，不如将其永久修改为ON DELETE SET NULL。这将完成类似的事情，你不需要打开和关闭键检查。像这样:

ALTER TABLE tablename1 DROP FOREIGN KEY fk_name1; //get rid of current constraints
ALTER TABLE tablename2 DROP FOREIGN KEY fk_name2;

ALTER TABLE tablename1 
  ADD FOREIGN KEY (table2_id) 
        REFERENCES table2(id)
        ON DELETE SET NULL  //add back constraint

ALTER TABLE tablename2 
  ADD FOREIGN KEY (table1_id) 
        REFERENCES table1(id)
        ON DELETE SET NULL //add back other constraint

看看这个(http://dev.mysql.com/doc/refman/5.5/en/alter-table.html)和这个(http://dev.mysql.com/doc/refman/5.5/en/create-table-foreign-keys.html)。

2013-03-19 14:33:29

其他回答

在phpMyAdmin中，您可以选择多行，然后单击删除操作。您将进入一个列出删除查询的屏幕。它是这样的:

请取消选中“启用外键检查”复选框，然后单击“是”执行它们。

这将使您能够删除行，即使有一个ON delete限制约束。

2015-12-31 10:57:47

当我想要截断一个表时，我通常只禁用外键约束，因为我一直回到这个答案，这是为未来的我:

SET FOREIGN_KEY_CHECKS=0;
TRUNCATE TABLE table;
SET FOREIGN_KEY_CHECKS=1;

2015-03-25 16:48:42

将外键约束设置为0并不是一个好主意，因为如果这样做，数据库将不能确保它没有违反引用完整性。这可能导致不准确、误导或不完整的数据。

使用外键是有原因的:因为子列中的所有值都应该与父列中的值相同。如果没有外键约束，子行可以有不在父行中的值，这将导致不准确的数据。

For instance, let's say you have a website for students to login and every student must register for an account as a user. You have one table for user ids, with user id as a primary key; and another table for student accounts, with student id as a column. Since every student must have a user id, it would make sense to make the student id from the student accounts table a foreign key that references the primary key user id in the user ids table. If there are no foreign key checks, a student could end up having a student id and no user id, which means a student can get an account without being a user, which is wrong.

想象一下，如果它发生在大量数据上。这就是为什么你需要外键检查。

最好弄清楚是什么导致了错误。最有可能的情况是，您试图从父行删除而不删除子行。尝试先从子行删除，再从父行删除。

2015-01-14 10:21:37

尝试禁用键或