最近在做项目的数据库迁移，从Oracle到Mysql，一个外键约束在Oracle运行正常，在mysql报异常。（因为才接手没几天，对业务和框架不熟，在处理问题时花了很多时间。）

[2018-08-01 13:34:19] [23000][1452] Cannot add or update a child row: a foreign key constraint fails (`bov`.`PRO_SITES_BRANDREQUEST`, CONSTRAINT `AA` FOREIGN KEY (`ID`) REFERENCES `PRO_SITES_SETUPREQUEST` (`ID`) ON DELETE CASCADE)

drop table Models;CREATE TABLE Models(ModelID number(6)PRIMARY KEY,Name VARCHAR(40));drop table Orders;CREATE TABLE Orders(ModelID number(8) PRIMARY KEY,Description VARCHAR(40),FOREIGN KEY (ModelID) REFERENCES Models (ModelID)ON DELETE cascade);insert into Models(ModelID, Name) values (1,'model');insert into Orders(ModelID,Description) values (1,'order');

select * from Models;1modelselect * from Orders;1order

drop table Models;CREATE TABLE Models(ModelID decimal(6,0)PRIMARY KEY,Name VARCHAR(40));drop table Orders;CREATE TABLE Orders(ModelID decimal(8,0) PRIMARY KEY,Description VARCHAR(40),FOREIGN KEY (ModelID) REFERENCES Models (ModelID)ON DELETE cascade);insert into Models(ModelID, Name) values (1,'model');insert into Orders(ModelID,Description) values (1,'order');

在执行最后一句时，报异常

[2018-08-01 14:06:16] [23000][1452] Cannot add or update a child row: a foreign key constraint fails (`bov`.`Orders`, CONSTRAINT `Orders_ibfk_1` FOREIGN KEY (`ModelID`) REFERENCES `Models` (`ModelID`) ON DELETE CASCADE)

原因：Models的ModelID是decimal(6,0)，而Orders的ModelID是decimal(8,0)，两个通过外键相连。因为类型不一致，mysql就不会认为其一定不等，而oracle可以做到不同类型的相容判等。

drop table Orders;CREATE TABLE Orders(ModelID decimal(6,0) PRIMARY KEY,Description VARCHAR(40),FOREIGN KEY (ModelID) REFERENCES Models (ModelID)ON DELETE cascade);insert into Orders(ModelID,Description) values (1,'order');

select * from Models;1modelselect * from Orders;1order

Mysql的外键约束设计有缺陷，如果不同单位的字段一定不同，应在添加FOREIGN KEY就报异常，而不是模棱两可的因为类型不同,但实际数值相等，其判断为不等于。

数据库表维护的时候，不同table中，意义相同的column，类型一定要保持一致。