MyBatis一对多，多对一和多对多联表查询-白红宇

MyBatis一对多，多对一和多对多联表查询

阅读量：4048 次

发布时间：2019-05-25

本文共 7911 字，大约阅读时间需要 26 分钟。

MyBatis中的一对多和对多一，多对多

主要就是resultMap中

association – 一个复杂的类型关联;许多结果将包成这种类型（一对多）

collection – 复杂类型的集合（多对一）

这2个属性的使用，而一对多和多对一都是相互的，只是站的角度不同。

实例：

这个实例只说这2个属性的使用方法，具体的配置和运行结果在文章中写出。

1.首先是数据库，沿用之前员工管理系统的数据库设计，上面是之前写过的一篇数据库设计，包含sql语句。

2.实体类：

我这里只列要实现映射功能用到的类user，role，department。department和user，是1对多关系，1个部门拥有多个员工。

role和user是多对多关系，1个角色可能有多个员工，而1个员工也可能有多个角色。而要实现多对多，在程序中则是拆分成2个一对多，详情见下面的实体类注释。

1）User.java

public class User {
       private int user_id;    private String user_name;    private String user_gender;    private String user_email;    private String user_phone;    private String user_address;    private Date user_birthday;    private int department_id;    //一对多，1个用户可能有多个角色。    private List
   
     roles;    //多对一，1个部门多个员工    private Department department;    public User() {        super();    }    public User(String user_name, String user_gender,            String user_email, String user_phone, String user_address,            Date user_birthday, int department_id) {        super();        this.user_name = user_name;        this.user_gender = user_gender;        this.user_email = user_email;        this.user_phone = user_phone;        this.user_address = user_address;        this.user_birthday = user_birthday;        this.department_id = department_id;    }    public int getUser_id() {        return user_id;    }    public void setUser_id(int user_id) {        this.user_id = user_id;    }    public String getUser_name() {        return user_name;    }    public void setUser_name(String user_name) {        this.user_name = user_name;    }    public String getUser_gender() {        return user_gender;    }    public void setUser_gender(String user_gender) {        this.user_gender = user_gender;    }    public String getUser_email() {        return user_email;    }    public void setUser_email(String user_email) {        this.user_email = user_email;    }    public String getUser_phone() {        return user_phone;    }    public void setUser_phone(String user_phone) {        this.user_phone = user_phone;    }    public String getUser_address() {        return user_address;    }    public void setUser_address(String user_address) {        this.user_address = user_address;    }    public Date getUser_birthday() {        return user_birthday;    }    public void setUser_birthday(Date user_birthday) {        this.user_birthday = user_birthday;    }    public int getDepartment_id() {        return department_id;    }    public void setDepartment_id(int department_id) {        this.department_id = department_id;    }    public Department getDepartment() {        return department;    }    public void setDepartment(Department department) {        this.department = department;    }    public List
    
      getRoles() {        return roles;    }    public void setRoles(List
     
       roles) {        this.roles = roles;    }    @Override    public String toString() {        return "User [user_id=" + user_id + ", user_name=" + user_name                + ", user_gender=" + user_gender + ", user_email=" + user_email                + ", user_phone=" + user_phone + ", user_address="                + user_address + ", user_birthday=" + user_birthday                + ", department_id=" + department_id + ", roles=" + roles                + ", department=" + department + "]\n";    }} 
 
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2）Role.java

public class Role {
       private int role_id;    private String role_name;    //一对多，1个角色也可能被多个用户拥有,所以是list    private List
   
     users;    public Role() {    }    public Role(int role_id, String role_name) {        super();        this.role_id = role_id;        this.role_name = role_name;    }    public int getRole_id() {        return role_id;    }    public void setRole_id(int role_id) {        this.role_id = role_id;    }    public String getRole_name() {        return role_name;    }    public void setRole_name(String role_name) {        this.role_name = role_name;    }    public List
    
      getUsers() {        return users;    }    public void setUsers(List
     
       users) {        this.users = users;    }    @Override    public String toString() {        return "Role [role_id=" + role_id + ", role_name=" + role_name                + ", users=" + users + "]";    }} 
 
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3）Department.java

public class Department {
       private int department_id;    private String department_name;    //1对多，1个部门多个用户    private List
   
     users;    public Department() {        super();    }    public Department(int department_id, String department_name,            List
    
      users) {        super();        this.department_id = department_id;        this.department_name = department_name;        this.users = users;    }    public int getDepartment_id() {        return department_id;    }    public void setDepartment_id(int department_id) {        this.department_id = department_id;    }    public String getDepartment_name() {        return department_name;    }    public void setDepartment_name(String department_name) {        this.department_name = department_name;    }    public List
     
       getUsers() {        return users;    }    public void setUsers(List
      
        users) {        this.users = users;    }    @Override    public String toString() {        return "Department [department_id=" + department_id                + ", department_name=" + department_name + ", users=" + users                + "]"+"\n";    }} 
 
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3.实体类的mapper文件

1）UserDepartmentMapper.xml，多对一，多个user对应1个department，使用association

2）UserMapper.xml，一对多，使用collection，1个user对应多个role，同时也是多对多中user这一方的一对多。如果要实现多对多，还需要完成1个role对应多个user,即完成2个1对多，写法同这里，就不在赘述。

不管是一对多还是多对一，还是多对多，只需要知道这2个属性哪个代表多哪个代表1，就可以很好的在实体的mapper文件中配置出来。

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