MySQL 1-many 关系问题

Question

我有 2 张表，一张用于员工，一张用于部门。一名员工只能在一个部门工作，但一个部门可以有多名员工。我在它们之间添加了一个外键约束，但是如果我尝试将具有相同数据的相同员工添加到同一部门，则查询将执行而不是失败。

create table Department(id int primary key auto_increment, name varchar(100));

create table Employee(id int primary key auto_increment, 
                 department_id int not null,
                 name varchar(100),
                 foreign key (department_id) references department(id));

INSERT INTO Department(name) VALUES('China');
INSERT INTO Department(name) VALUES('England');

INSERT INTO Employee(department_id,name) VALUES (1,'Mark');
INSERT INTO Employee(department_id,name) VALUES (1,'Mark');

第二次插入 Employee 'Mark' 后，我期待一个错误，但查询总是执行。如何限制一名特定员工只能在一个部门工作？

Answer 1

您需要在 (department_id, name) 上建立唯一索引。

这仍然允许在其他部门内使用姓名标记。

ALTER TABLE `employee` ADD UNIQUE INDEX `unique_department_id__name` (`department_id`, `name`);

查询

INSERT INTO Employee(department_id,NAME) VALUES (1,'Mark');
INSERT INTO Employee(department_id,NAME) VALUES (1,'Mark');

结果

Query: INSERT INTO Employee(department_id,name) VALUES (1,'Mark')

1 row(s) affected

Execution Time : 0.013 sec
Transfer Time  : 0 sec
Total Time     : 0.014 sec
-----------------------------------------------------------

Query: INSERT INTO Employee(department_id,name) VALUES (1,'Mark')

Error Code: 1062
Duplicate entry '1-Mark' for key 'unique_department_id__name'

Execution Time : 0 sec
Transfer Time  : 0 sec
Total Time     : 0.003 sec

已编辑

查询

INSERT INTO Employee(department_id,NAME) VALUES (2,'Mark');

结果

Query: INSERT INTO Employee(department_id,NAME) VALUES (2,'Mark')

1 row(s) affected

Execution Time : 0.021 sec
Transfer Time  : 0 sec
Total Time     : 0.022 sec

Answer 2

发生这种情况是因为名称列上没有约束。

添加唯一约束 Employee.name 。

例如

ALTER TABLE Employee
ADD CONSTRAINT UC_Person UNIQUE (name)

; ,