“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.

有一个很好的理由不使用它们:如果你不了解它们的作用或如何使用它们。

在错误的情况下，外键约束可能导致事故的瀑布式复制。如果有人删除了错误的记录，恢复它可能会成为一项艰巨的任务。

同样，相反，当您需要删除某些内容时，如果设计不当，约束可能会导致各种锁阻止您的操作。

我听到的观点是前端应该有这些业务规则。当您一开始就不应该允许任何破坏约束的插入时，外键会“增加不必要的开销”。我同意吗?不，但我一直都是这么听说的。

编辑:我猜他指的是外键约束，而不是外键这个概念。

我不得不在这里第二多的评论，外键是必要的项目，以确保你有完整的数据。ON DELETE和ON UPDATE的不同选项将允许你绕过一些人们在这里提到的关于它们的使用的“下降”。

我发现在我99%的项目中，我会使用FK来加强数据的完整性，然而，在很少的情况下，我的客户必须保留他们的旧数据，不管它有多糟糕....但后来我花了很多时间写代码，只得到有效的数据，所以它变得毫无意义。

@ emphasis——这正是导致维护噩梦的心态。

为什么，哦，为什么要忽略声明性引用完整性(其中数据至少可以保证一致)，而支持所谓的“软件强制”，这充其量是一种薄弱的预防措施。

我同意前面的答案，因为它们对维护数据一致性很有用。然而，Jeff Atwood几周前发表了一篇有趣的文章，讨论了规范化和一致性数据的利弊。

简而言之，在处理大量数据时，非规范化数据库可以更快;你可能不关心精确的一致性取决于应用程序，但它迫使你在处理数据时更加小心，因为DB不会。