跨应用程序生命周期的可维护性和稳定性如何?大多数数据的生命周期都比使用它的应用程序长。关系和数据完整性非常重要，不能寄希望于下一个开发团队在应用程序代码中做出正确的处理。如果您没有使用过不尊重自然关系的脏数据的db，那么您将会使用。数据完整性的重要性将变得非常清楚。

验证外键约束需要一些CPU时间，因此有些人省略外键以获得额外的性能。

对我来说，如果您想要遵循ACID标准，那么拥有外键以确保引用完整性是至关重要的。

没有充分的理由不使用它们……除非孤行对你来说不是什么大问题。

“在添加记录之前，检查对应的记录是否存在于另一个表中”是业务逻辑。

这里有一些你不希望在数据库中使用它的原因:

If the business rules change, you have to change the database. The database will need to recreate the index in a lot of cases and this is slow on large tables. (Changing rules include: allow guests to post messages or allow users to delete their account despite having posted comments, etc). Changing the database is not as easy as deploying a software fix by pushing the changes to the production repository. We want to avoid changing the database structure as much as possible. The more business logic there is in the database the more you increase the chances of needing to change the databae (and triggering re-indexing). TDD. In unit tests you can substitute the database for mocks and test the functionality. If you have any business logic in your database, you are not doing complete tests and would need to either test with the database or replicate the business logic in code for testing purposes, duplicating the logic and increasing the likelyhood of the logic not working in the same way. Reusing your logic with different data sources. If there is no logic in the database, my application can create objects from records from the database, create them from a web service, a json file or any other source. I just need to swap out the data mapper implementation and can use all my business logic with any source. If there is logic in the database, this isn't possible and you have to implement the logic at the data mapper layer or in the business logic. Either way, you need those checks in your code. If there's no logic in the database I can deploy the application in different locations using different database or flat-file implementations.

更新:我现在总是使用外键。对于反对意见“他们使测试变得复杂”，我的回答是“编写单元测试，这样他们就根本不需要数据库。任何使用该数据库的测试都应该正确地使用它，这包括外键。如果准备工作很痛苦，那就找一种不那么痛苦的方式来做。”

外键使自动化测试复杂化

假设您正在使用外键。您正在编写一个自动测试，该测试表示“当我更新财务帐户时，它应该保存交易记录。”在这个测试中，您只关心两个表:帐户和事务。

但是，accounts对契约有一个外键，契约对客户有一个fk，客户对城市有一个fk，城市对州有一个fk。

现在，数据库将不允许您运行测试，除非在四个与测试无关的表中设置数据。

至少有两种可能的观点:

“这是一件好事:你的测试应该是现实的，这些数据限制将存在于生产中。” “这是一件坏事:你应该能够在不涉及其他部分的情况下对系统的各个部分进行单元测试。您可以为整个系统添加集成测试。”

也可以在运行测试时暂时关闭外键检查。至少MySQL支持这一点。