我相信有很多应用程序可以使用它，但这不是最好的主意。您不能总是指望您的应用程序能够正确地管理数据库，坦白地说，管理数据库不应该是您的应用程序非常关心的问题。

如果您正在使用关系数据库，那么似乎应该在其中定义一些关系。不幸的是，这种态度(您不需要外键)似乎被许多应用程序开发人员所接受，他们宁愿不为数据完整性等愚蠢的事情所困扰(但需要这样做，因为他们的公司没有专门的数据库开发人员)。通常在这些类型的数据库中，你很幸运只有主键;)

使用外键的其他原因: —可以更好地重用数据库

不使用外键的其他原因: —您试图通过减少重用来锁定客户。

使用外键的原因:

you won't get Orphaned Rows you can get nice "on delete cascade" behavior, automatically cleaning up tables knowing about the relationships between tables in the database helps the Optimizer plan your queries for most efficient execution, since it is able to get better estimates on join cardinality. FKs give a pretty big hint on what statistics are most important to collect on the database, which in turn leads to better performance they enable all kinds of auto-generated support -- ORMs can generate themselves, visualization tools will be able to create nice schema layouts for you, etc. someone new to the project will get into the flow of things faster since otherwise implicit relationships are explicitly documented

不使用外键的原因:

you are making the DB work extra on every CRUD operation because it has to check FK consistency. This can be a big cost if you have a lot of churn by enforcing relationships, FKs specify an order in which you have to add/delete things, which can lead to refusal by the DB to do what you want. (Granted, in such cases, what you are trying to do is create an Orphaned Row, and that's not usually a good thing). This is especially painful when you are doing large batch updates, and you load up one table before another, with the second table creating consistent state (but should you be doing that sort of thing if there is a possibility that the second load fails and your database is now inconsistent?). sometimes you know beforehand your data is going to be dirty, you accept that, and you want the DB to accept it you are just being lazy :-)

我认为(我不确定!)大多数已建立的数据库都提供了一种指定外键的方法，这种方法不是强制的，只是一些元数据。由于不强制执行消除了不使用fk的所有理由，如果第二部分中的任何理由适用，您可能应该走那条路。

One time when an FK might cause you a problem is when you have historical data that references the key (in a lookup table) even though you no longer want the key available. Obviously the solution is to design things better up front, but I am thinking of real world situations here where you don't always have control of the full solution. For example: perhaps you have a look up table customer_type that lists different types of customers - lets say you need to remove a certain customer type, but (due to business restraints) aren't able to update the client software, and nobody invisaged this situation when developing the software, the fact that it is a foreign key in some other table may prevent you from removing the row even though you know the historical data that references it is irrelevant. After being burnt with this a few times you probably lean away from db enforcement of relationships. (I'm not saying this is good - just giving a reason why you may decide to avoid FKs and db contraints in general)

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.