六、多对多查询

实体类：

public class Role implements Serializable {

    private Integer roleId;
    private String roleName;
    private String roleDesc;

    //多对多的关系映射：一个角色可以赋予多个用户
    private List<User> users;

    @Override
    public String toString() {
        return "Role{" +
                "roleId=" + roleId +
                ", roleName='" + roleName + '\'' +
                ", roleDesc='" + roleDesc + '\'' +
                '}';
    }
}

public class User implements Serializable {

    private Integer id;
    private String username;
    private String address;
    private String sex;
    private Date birthday;

    //多对多的关系映射：一个用户可以具备多个角色
    private List<Role> roles;

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", address='" + address + '\'' +
                ", sex='" + sex + '\'' +
                ", birthday=" + birthday +
                '}';
    }
}

测查询所有角色，同时获取每个角色的用户信息

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.liaoxiang.dao.IRoleDao">

    <!--定义role表的ResultMap, 查询所有角色，并显示角色对应的有哪些人-->
    <resultMap id="roleMap" type="role">
        <id property="roleId" column="rid"></id>
        <result property="roleName" column="role_name"></result>
        <result property="roleDesc" column="role_desc"></result>
        <collection property="users" ofType="user">
            <id column="id" property="id"></id>
            <result column="username" property="username"></result>
            <result column="address" property="address"></result>
            <result column="sex" property="sex"></result>
            <result column="birthday" property="birthday"></result>
        </collection>
    </resultMap>

    <!--查询所有-->
    <select id="findAll" resultMap="roleMap">
        select u.*,r.id as rid,r.role_name,r.role_desc from role r
        left outer join user_role ur  on r.id = ur.rid
        left outer join user u on u.id = ur.uid
    </select>
</mapper>

@Test
public void testFindAll(){
    List<Role> roles = roleDao.findAll();
    for(Role role : roles){
        System.out.println("---------------每个角色的信息-------------");
        System.out.println(role);
        System.out.println(role.getUsers());
    }
}

角色信息：Role{roleId=1, roleName='院长', roleDesc='管理整个学院'}
用户信息：[User{id=41, username='张三', address='绵阳', sex='男', birthday=Tue Feb 27 17:47:08 CST 2018},
         User{id=45, username='李四', address='广州', sex='男', birthday=Sun Mar 04 12:04:06 CST 2018}]

角色信息：Role{roleId=2, roleName='总裁', roleDesc='管理整个公司'}
用户信息：[User{id=41, username='张三', address='绵阳', sex='男', birthday=Tue Feb 27 17:47:08 CST 2018}]

角色信息：Role{roleId=3, roleName='校长', roleDesc='管理整个学校'}
用户信息：[]

查询用户信息，同时查询出每个用户的角色信息

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.liaoxiang.dao.IUserDao">

    <!-- 定义User的resultMap，查询用户，并且查询出用户的角色信息-->
    <resultMap id="userMap" type="user">
        <id property="id" column="id"></id>
        <result property="username" column="username"></result>
        <result property="address" column="address"></result>
        <result property="sex" column="sex"></result>
        <result property="birthday" column="birthday"></result>
        <!-- 配置角色集合的映射 -->
        <collection property="roles" ofType="role">
            <id property="roleId" column="rid"></id>
            <result property="roleName" column="role_name"></result>
            <result property="roleDesc" column="role_desc"></result>
        </collection>
    </resultMap>

    <!-- 查询所有 -->
    <select id="findAll" resultMap="userMap">
        select u.*,r.id as rid,r.role_name,r.role_desc from user u
        left outer join user_role ur  on u.id = ur.uid
        left outer join role r on r.id = ur.rid
    </select>
    
</mapper>

@Test
public void testFindAll(){
    List<User> users = userDao.findAll();
    for(User user : users){
        System.out.println("用户信息："+user);
        System.out.println("用户角色："+user.getRoles());
        System.out.println("===================每个用户信息================");
    }
}
用户信息：User{id=41, username='张三', address='绵阳', sex='男', birthday=Tue Feb 27 17:47:08 CST 2018}
用户角色：[Role{roleId=1, roleName='院长', roleDesc='管理整个学院'},
		 Role{roleId=2, roleName='总裁', roleDesc='管理整个公司'}]

用户信息：User{id=45, username='李四', address='广州', sex='男', birthday=Sun Mar 04 12:04:06 CST 2018}
用户角色：[Role{roleId=1, roleName='院长', roleDesc='管理整个学院'}]

用户信息：User{id=46, username='老王', address='北京', sex='男', birthday=Wed Mar 07 17:37:26 CST 2018}
用户角色：[]

用户信息：User{id=49, username='王五', address='*', sex='女', birthday=Mon Jun 03 16:40:00 CST 2019}
用户角色：[]