问题描述
关于如何使用 MySQL 的外键构造有什么好的解释吗?
Is there any good explanation of how to use MySQL's foreign key construct?
我并不能完全从 MySQL 文档本身中得到它.到目前为止，我一直在处理诸如外键连接和编程代码之类的事情.
I don't quite get it from the MySQL docs themselves. Up until now I've been handling things like foreign keys with joins and programming code.
问题的第二部分，使用 MySQL 的内置外键是否有任何改进?
And the second part of the question, are there any improvements to be made by using MySQL's inbuilt foreign keys?
推荐答案
FOREIGN KEYS只要保证你的数据一致即可.
FOREIGN KEYS just ensure your data are consistent.
它们并没有提高查询的效率，它们只是让一些错误的查询失败.
They do not improve queries in sense of efficiency, they just make some wrong queries fail.
如果你有这样的关系:
CREATE TABLE department (id INT NOT NULL)
CREATE TABLE employee (id INT NOT NULL, dept_id INT NOT NULL, FOREIGN KEY (dept_id) REFERENCES department(id))
，那么如果一个department有一些employee，你就不能删除它.
, then you cannot delete a department if it has some employee's.
如果您将 ON DELETE CASCADE 提供给 FOREIGN KEY 定义，则引用行将与被引用行一起自动删除.
If you supply ON DELETE CASCADE to the FOREIGN KEY definition, the referencing rows will be deleted automatically along with the referenced ones.
作为一个约束，FOREIGN KEY 实际上会稍微减慢查询速度.
As a constraint, FOREIGN KEY actually slows down the queries a little.
从引用表中删除或插入引用表时需要进行额外检查.
Extra checking needs to be performed when deleting from a referenced table or inserting into a referencing one.
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