@ 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。

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.

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;)

“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.

澄清数据库是一个没有主键或外键的商业数据库示例。

http://www.geekinterview.com/question_details/18869

有趣的是，技术文档花了很大的篇幅来解释表是如何关联的，用什么列来连接它们等等。

换句话说，它们本可以使用显式声明(DRI)连接表，但它们选择不这样做。

因此，澄清数据库充满了不一致，性能不佳。

但我认为它使开发人员的工作更容易，不必编写代码来处理引用完整性，例如在删除或添加之前检查相关行。

我认为，这就是关系数据库中没有外键约束的主要好处。它使开发变得更容易，至少从不顾一切的角度来看是这样。