更大的问题是:你会戴着眼罩开车吗?如果你开发一个没有参考约束的系统就是这样。请记住，业务需求会改变，应用程序设计会改变，代码中相应的逻辑假设也会改变，逻辑本身也会被重构，等等。一般来说，数据库中的约束是在当代逻辑假设下放置的，对于特定的逻辑断言和假设集似乎是正确的。

在应用程序的整个生命周期中，引用检查和数据检查约束控制通过应用程序收集的数据，特别是当新需求驱动逻辑应用程序更改时。

从实时事务处理系统的角度来看，外键本身不会“提高性能”，也不会显著“降低性能”。然而，在大容量“批处理”系统中，约束检查存在一个聚合代价。这就是实时和批量事务处理的区别;批处理——通过约束检查，按顺序处理的批处理的总成本会对性能造成影响。

在一个设计良好的系统中，数据一致性检查将在处理批处理之前完成(然而，这里也有相关的成本);因此，加载时不需要外键约束检查。事实上，所有的约束，包括外键，都应该暂时禁用，直到批处理完成。

QUERY性能——如果表是在外键上连接的，那么要认识到外键列不是索引的事实(尽管根据定义，各自的主键是索引的)。通过索引一个外键，也就是索引任何键，在索引上连接表有助于提高性能，而不是通过连接一个带外键约束的非索引键。

Changing subjects, if a database is just supporting website display/rendering content/etc and recording clicks, then a database with full constraints on all tables is over kill for such purposes. Think about it. Most websites don’t even use a database for such. For similar requirements, where data is just being recorded and not referenced per say, use an in-memory database, which does not have constraints. This doesn’t mean that there is no data model, yes logical model, but no physical data model.

“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.

我只知道Oracle数据库，不知道其他数据库，而且我知道外键对于保持数据完整性是必不可少的。在插入数据之前，需要建立一个数据结构，并且建立正确的数据结构。当这一步完成时——所有的主键和外键都创建好了——工作就完成了!

意思是:孤立的行?不。这辈子都没见过。除非一个糟糕的程序员忘记了外键，或者他在另一个层次上实现了外键。在Oracle的环境中，这两者都是巨大的错误，会导致数据复制、孤儿数据，从而导致数据损坏。我无法想象一个没有强制FK的数据库。在我看来是一片混乱。这有点像Unix权限系统:假设每个人都是root用户。想想混乱吧。

外键是必不可少的，就像主键一样。这就像是说:如果我们移除主键会怎样?那么，整个混乱将会发生。这是什么。不能将主键或外键的职责移到编程级别，但必须移到数据级别。

缺点呢?是的，当然!因为在插入时，会有更多的检查。但是，如果数据完整性比性能更重要，那么这是显而易见的。Oracle上的性能问题更多地与索引有关，索引包含PK和FK。

这是一个教养问题。如果在您的教育或职业生涯中，您曾花时间维护数据库(或与有才华的人密切合作)，那么实体和关系的基本原则就会在您的思维过程中根深蒂固。这些基本知识包括如何/何时/为什么在数据库中指定键(主键、外键和备选键)。这是第二天性。

If, however, you've not had such a thorough or positive experience in your past with RDBMS-related endeavors, then you've likely not been exposed to such information. Or perhaps your past includes immersion in an environment that was vociferously anti-database (e.g., "those DBAs are idiots - we few, we chosen few java/c# code slingers will save the day"), in which case you might be vehemently opposed to the arcane babblings of some dweeb telling you that FKs (and the constraints they can imply) really are important if you'd just listen.

大多数人小时候都被教育刷牙很重要。没有它你能过吗?当然，但在某个时候，如果你每顿饭后都刷牙，那么你的牙齿就会减少。如果妈妈和爸爸们有足够的责任心，把数据库设计和口腔卫生都包括在内，我们就不会有这样的对话了。: -)

Wowowo... Answers everywhere. Actually this is the most complicated topic I have ever encountered. I use FKs when they are needed but on production environment I rarely use them. Here is my whys I rarely use the Fks: 1. Most of the time I am dealing with huge data on small server to improve performance I need to remove the FKs. Because when you have FKs and you do Create, Update or Delete the RDBMS first check if there no constraint violation and if you have huge DB that could be something fatal 2. Sometimes I need to import data from others places and because I am not too sure of how well structured they are, I simply drop the FKs. 3. In case you are dealing with multiple DBs and having reference key in an other DB will not go well(as for now) until you remove the FKs (cross database relations) 4. They was also a case when you write an application which will seat on whatever RDBMS or you want your DB to be exported and imported in any RDBMS system in this case each specific RDBMS system has his own way of dealing with FKs and you will probably be obliged to drop the use of FKs. 5. If you user RDBMS platform (ORMs) you know that some of them offer their own mapping depending on the solution and technicality their offer and you don't care about creating the tables and their FKs. 6. Before the last point will be knowledge to deal with DB that has FKs and the knowledge to write an application that does all the Job without the need of FK 7. Lastly as I started saying it all depend on your scenario, in case knowledge is not a barrier. You will always want to run the best of the best you can get!

谢谢大家!

Quite often we receive the errors with FK constraints Cannot add or update a child row: a foreign key constraint fails Suppose there are two tables inventory_source and contract_lines, and we are referring inventory_source_id in contract_lines from inventory_source and suppose we want to delete record from inventory_source and the record is already present in contract_lines or we want to delete the PK column from Base table, we get errors for FK constraints, we can avoid it using the steps jotted below.

CREATE TABLE inventory_source (
inventory_source_id int(11) NOT NULL AUTO_INCREMENT,
display_name varchar(40) NOT NULL,
state_id int(11) NOT NULL,
PRIMARY KEY (inventory_source_id),
KEY state_id (state_id),
CONSTRAINT ba_inventory_source_state_fk FOREIGN KEY (state_id) REFERENCES   ba_state (state_id)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

CREATE TABLE contract_lines(
contract_line_id int(11) NOT NULL AUTO_INCREMENT,
inventory_source_id int(11) NULL ,
PRIMARY KEY (contract_line_id),
UNIQUE KEY contract_line_id (contract_line_id),
KEY AI_contract_line_id (contract_line_id),
KEY contract_lines_inventory_source_fk (inventory_source_id),
CONSTRAINT contract_lines_inventory_source_fk FOREIGN KEY       (inventory_source_id) REFERENCES ba_inventory_source (inventory_source_id)
) ENGINE=InnoDB AUTO_INCREMENT=135 DEFAULT CHARSET=utf8 ;

我们可以采用以下步骤克服它:-

Delete or update the row from the inventory_source will automatically delete or update the matching rows in the contract_lines table and this is known as cascade delete or update. Another way of doing it is setting the column i.e inventory_source_id in the contract_lines table to NULL, when a record corresponding to it is deleted in the inventory_source table. We can restrict the parent table for delete or update in other words one can reject the delete or update operation for the inventory_source table. Attempt to delete or update a primary key value will not be permitted to proceed if there is a related foreign key value in the referenced table.