MySQL数据库创建，多个外键错误？

Question

所以我试图在 mySQL 中创建一个数据库。我有一堆 create table 语句和一堆 alter table 语句。然而，大约一半的 alter table 语句随机给我 1215“无法添加外键约束”错误，这与医生告诉我骨折而不告诉我是哪一个一样有用。无论如何，这是失败的其中之一：

ALTER TABLE trip_instance ADD FOREIGN KEY (Trip_ID) REFERENCES signups(Trip_ID);

以下是该特定语句的相关创建语句。我确保指定列是主键。

CREATE TABLE Trip_Instance (Instance_ID INT, Trip_Date DATE, Trip_ID INT, Employee_Leader VARCHAR(50), Employee_assistant VARCHAR(50),PRIMARY KEY(Instance_ID, Trip_ID));
CREATE TABLE Signups (Customer_ID INT, Trip_ID INT, Insurance_Form BOOLEAN, PRIMARY KEY (Customer_ID, Insurance_Form, Trip_ID));

编辑： 所以我用了：

set foreign_key_checks=0;

在不知道它的作用的情况下，我现在得到一堆关于索引的 1822/1825 错误代码，而不是错误代码 1215，但即使我在创建表语句中添加索引语句，它也无法告诉我相同的信息问题。

我只是把我所有的陈述都写在下面，也许我遗漏了一些重要的东西：

CREATE DATABASE OAG_Club;
USE OAG_Club;
CREATE TABLE Customer (customer_ID INT, Name VARCHAR(50), Home_Phone VARCHAR(10), Work_Phone VARCHAR(10), DOB DATE, Address VARCHAR(50), Customer_Type VARCHAR(50), Sponsor_ID INT, PRIMARY KEY(customer_ID));

CREATE TABLE Signups (Customer_ID INT, Trip_ID INT, Insurance_Form BOOLEAN, PRIMARY KEY (Customer_ID, Insurance_Form, Trip_ID));

CREATE TABLE Trip_Instance (Instance_ID INT, Trip_Date DATE, Trip_ID INT, Employee_Leader VARCHAR(50), Employee_assistant VARCHAR(50),PRIMARY KEY(Instance_ID, Trip_ID));

CREATE TABLE Trip_Type (Trip_ID INT, Trip_Name VARCHAR(50), DIFF_lvl INT, Trip_Fee INT, Trip_Length INT, PRIMARY KEY (Trip_ID));

CREATE TABLE Rental_Agreement (Agreement_Number INT, Start_Date DATE, customer_ID INT, employee_ID INT, PRIMARY KEY (Agreement_Number));

CREATE TABLE Rental_Detail (Expected_Return DATE, Real_Return DATE, item_number INT, agreement_num INT);

CREATE TABLE Inventory (Item_Number INT, `Condition` VARCHAR(50), equip_name VARCHAR(50), PRIMARY KEY (Item_Number));

CREATE TABLE Equipment (Equip_Name VARCHAR(50), Student_Fee FLOAT, FacStaffAl_Fee FLOAT, Guest_Fee FLOAT, PRIMARY KEY (Equip_Name));

CREATE TABLE OAG_Employee (Employee_ID INT, Employee_Name VARCHAR(50), Start_Date DATE, End_Date DATE, Position_ID INT, PRIMARY KEY (Employee_ID));

CREATE TABLE Position (Position_ID INT, Position_Descr VARCHAR(50), Position_Salary FLOAT, PRIMARY KEY (Position_ID));

set foreign_key_checks=0;
ALTER TABLE inventory ADD FOREIGN KEY (equip_name) REFERENCES equipment(equip_name);
ALTER TABLE customer ADD FOREIGN KEY (sponsor_ID) REFERENCES customer(sponsor_ID);*/Nope*/
ALTER TABLE signups ADD FOREIGN KEY (customer_ID) REFERENCES customer(customer_ID);
ALTER TABLE signups ADD FOREIGN KEY (Trip_ID) REFERENCES Trip_Instance(Trip_ID); /*nope*/
ALTER TABLE trip_instance ADD FOREIGN KEY (Trip_ID) REFERENCES signups(Trip_ID); /*nope*/
ALTER TABLE trip_instance ADD FOREIGN KEY (Employee_Leader) REFERENCES OAG_Employee(Employee_ID); /*nope*/
ALTER TABLE trip_instance ADD FOREIGN KEY (Employee_Assistant) REFERENCES OAG_Employee(Employee_ID); /*nope*/
ALTER TABLE rental_agreement ADD FOREIGN KEY (customer_id) REFERENCES customer(customer_id);
ALTER TABLE rental_agreement ADD FOREIGN KEY (employee_id) REFERENCES oag_employee(employee_id);
ALTER TABLE rental_detail ADD FOREIGN KEY (item_number) REFERENCES inventory(item_number);
ALTER TABLE rental_detail ADD FOREIGN KEY (agreement_num) REFERENCES rental_agreement(agreement_num); /*nope*/
ALTER TABLE oag_employee ADD FOREIGN KEY (position_ID) REFERENCES `position`(position_ID);`

Answer 1

表中的外键必须指向另一个表中的主键，而不仅仅是主键的一部分。因此，在您的表 Signups 中，主键是由三列组成的复合 PK。另一个表中的外键必须包含等效的三列。一般来说，我认为最好设置单列PK，然后在其他表中定义单列FK更容易。如果需要，请务必在三列上设置唯一约束，但有一个单独的标识列作为 PK。

所以对于那些它抱怨的人：

ALTER TABLE customer ADD FOREIGN KEY (sponsor_ID) REFERENCES customer(sponsor_ID);
*/Nope - FK column is not referencing a PK column. PK column in customer is customer_ID. Is this a self-referencing table? If so then...*/
ALTER TABLE customer ADD FOREIGN KEY (sponsor_ID) REFERENCES customer(customer_ID);

ALTER TABLE signups ADD FOREIGN KEY (Trip_ID) REFERENCES Trip_Instance(Trip_ID);
/*nope - Trip_ID is the PK in table Trip_Type, not Trip_Instance, so...*/
ALTER TABLE signups ADD FOREIGN KEY (Trip_ID) REFERENCES Trip_Type(Trip_ID); 

ALTER TABLE trip_instance ADD FOREIGN KEY (Trip_ID) REFERENCES signups(Trip_ID);
/*nope - same as above, so...*/
ALTER TABLE trip_instance ADD FOREIGN KEY (Trip_ID) REFERENCES Trip_Type(Trip_ID);

ALTER TABLE trip_instance ADD FOREIGN KEY (Employee_Leader) REFERENCES OAG_Employee(Employee_ID);
ALTER TABLE trip_instance ADD FOREIGN KEY (Employee_Assistant) REFERENCES OAG_Employee(Employee_ID);
/*nope - for both of these the FK column is varchar(50) but the referenced column is INT. What you should have is...*/

CREATE TABLE Trip_Instance (Instance_ID INT, Trip_Date DATE, Trip_ID INT, Employee_Leader_ID INT, Employee_assistant_ID INT,PRIMARY KEY(Instance_ID, Trip_ID));

ALTER TABLE trip_instance ADD FOREIGN KEY (Employee_Leader_ID) REFERENCES OAG_Employee(Employee_ID); /*nope*/
ALTER TABLE trip_instance ADD FOREIGN KEY (Employee_Assistant_ID) REFERENCES OAG_Employee(Employee_ID);

希望这会有所帮助。