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.

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。

在我参与的一个项目中，经常存在隐式关系而不是显式关系，这样可以在同一列上连接多个表。

请看下面的表格

地址

AddressId (PK) EntityId EntityType 城市 状态 国家 等。

EntityType的值可能是Employee、Company、Customer, EntityId指的是您感兴趣的表的主键。

我真的不认为这是最好的方法，但它对这个项目有效。

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。

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。

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.