mysql

Question

我是 sql 的新手。

我在尝试创建外键时遇到此错误：

cannot add foreign key constraint

当我尝试创建订单时 table。这是我的代码：

drop database if exists Company;
create database Company;
use Company;
create table WORKERS(w_id varchar(4), w_name varchar(20) not null, telephone varchar(12), e_mail varchar(35) unique);
create table CUSTOMERS(customer_id varchar(4), customer_name varchar(20) not null, telephone varchar(12), e_mail varchar(35) unique, credit bool);
create table WAREHOUSE(m_id int unsigned primary key, describes varchar(20) not null, fl char(1));
create table ITEM(p_id int(4) unsigned primary key, p_name varchar(15) not null, p_price float not null);
create table INVENTORY(in_id int auto_increment primary key, m_id int unsigned not null, p_id int(4) unsigned not null, amount int not null,
foreign key (m_id) references WAREHOUSE(m_id),
foreign key (p_id) references ITEM(p_id)
);
create table ORDERS(o_id int auto_increment primary key, customer_id varchar(4),
p_id int(4) unsigned, w_id varchar(4), amount int unsigned not null, 
date_of_order date not null,
foreign key (p_id) references ITEM(p_id),
foreign key (w_id) references WORKERS(w_id),
foreign key (customer_id) references CUSTOMERS(customer_id)
);

我将订单（我遇到问题的地方）放在不同的行上，以便于您阅读。

我在这里搜索答案，但没有找到任何具体的答案来回答我的问题。

有人知道问题出在哪里吗？谢谢！

Answer 1

问题是您正在尝试创建与 WORKERS 和 CUSTOMERS 表中未设置为 [=14= 的字段的 FOREIGN KEY 关系].

FOREIGN KEY 需要指向 PRIMARY KEY。将您的创建脚本更改为以下内容：

create table WORKERS(w_id varchar(4) primary key, w_name varchar(20) not null, telephone varchar(12), e_mail varchar(35) unique);
create table CUSTOMERS(customer_id varchar(4) primary key, customer_name varchar(20) not null, telephone varchar(12), e_mail varchar(35) unique, credit bool);

不过请注意。我建议不要将 VARCHAR (4) 用作 PRIMARY KEY 字段。我建议改用 INT AUTO_INCREMENT。

Answer 2

引用的 table 的列应声明为 primary key 以分配外键。对于您的 orders table，WORKERS(w_id) 和 CUSTOMERS(customer_id) 未声明为主键，因此您会收到错误消息。

修改后的语句：

drop database if exists Company;
create database Company;
use Company;
create table WORKERS(w_id varchar(4) primary key, w_name varchar(20) not null, telephone varchar(12), e_mail varchar(35) unique);
create table CUSTOMERS(customer_id varchar(4) primary key, customer_name varchar(20) not null, telephone varchar(12), e_mail varchar(35) unique, credit bool);
create table WAREHOUSE(m_id int unsigned primary key, describes varchar(20) not null, fl char(1));
create table ITEM(p_id int(4) unsigned primary key, p_name varchar(15) not null, p_price float not null);
create table INVENTORY(in_id int auto_increment primary key, m_id int unsigned not null, p_id int(4) unsigned not null, amount int not null,
foreign key (m_id) references WAREHOUSE(m_id),
foreign key (p_id) references ITEM(p_id)
);

create table ORDERS(o_id int auto_increment primary key, customer_id varchar(4),
p_id int(4) unsigned, w_id varchar(4), amount int unsigned not null, 
date_of_order date not null,
foreign key (p_id) references ITEM(p_id),
foreign key (w_id) references WORKERS(w_id),
foreign key (customer_id) references CUSTOMERS(customer_id)
);

SQLFiddle Demo

mysql - 创建 table 时无法添加外键约束

mysql - cannot add foreign key constraint when creating a table

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