Mybatis的关联查询（association和collection）

关联查询

• 实体间的关系（拥有 has、属于 belong）

  • OneToOne：一对一关系（account ←→ user）

  • OneToMany：一对多关系（user ←→ account）

  • ManyToMany：多对多关系（user ←→ role）

• 什么是关联查询

  当访问关系的一方时，如果需要查看与之关联的另一方数据，则必须使用表链接查询，将查询到的另一方数据，保存在本方的属性中

• 关联查询的语法

  指定“一方”关系时（对象），使用< association javaType="" >

  指定“多方”关系时（集合），使用< collection ofType="" >

一，一对一查询

需求：查询账户信息，关联查询用户信息。

分析：因为一个账户信息只能供某个用户使用，所以从查询账户信息出发关联查询用户信息为一对一查询。

com.by.pojo下的Account类

public class Account implements Serializable {

    private Integer id;
    private Integer uid;
    private Double money;
    //加入User类的对象作为Account类的一个属性
    private User user;

    public User getUser() {
        return user;
    }
    public void setUser(User user) {
        this.user = user;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public Integer getUid() {
        return uid;
    }

    public void setUid(Integer uid) {
        this.uid = uid;
    }

    public Double getMoney() {
        return money;
    }

    public void setMoney(Double money) {
        this.money = money;
    }

    @Override
    public String toString() {
        return "Account{" +
                "id=" + id +
                ", uid=" + uid +
                ", money=" + money +
                ", user=" + user +
                '}';
    }
}

com.by.dao下的AcountDao接口

public interface AccountDao {
    List<Account> findAll();
}

com.by.dao下的AcountDao.xml文件

<?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.by.dao.AccountDao">
    <!-- 结果映射 -->
    <resultMap type="account" id="findAllResultMap">
        <id column="aid" property="id"/>
        <result column="uid" property="uid"/>
        <result column="money" property="money"/>
        <!-- 指定关系表中数据的封装规则 -->
        <association property="user" javaType="user">
            <id column="id" property="id"/>
            <result column="username" property="username"/>
            <result column="sex" property="sex"/>
            <result column="birthday" property="birthday"/>
            <result column="address" property="address"/>
        </association>
    </resultMap>
    <select id="findAll" resultMap="findAllResultMap">
        select u.*,a.id as aid,a.uid,a.money from account a,user u where a.uid =u.id
    </select>
</mapper>

测试类

private SqlSession sqlSession;
    private InputStream inputStream;
    @Before
    public void init() throws IOException {
        //加载配置文件
        String resource = "mybatis-config.xml";
        inputStream = Resources.getResourceAsStream(resource);
        //创建SessionFactory
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //使用数据的会话实例
        sqlSession = sessionFactory.openSession();
    }
@After
    public void close() throws IOException {
        sqlSession.close();
        inputStream.close();
    } 
@Test
    public void testOneToOne() {
        AccountDao accountDao = sqlSession.getMapper(AccountDao.class);
        List<Account> accountList = accountDao.findAll();
        for (Account ac : accountList) {
            System.out.println(ac);
        }
    }

输出结果

二，一对多查询

需求：查询所有用户信息及用户关联的账户信息。

分析：用户信息和他的账户信息为一对多关系，并且查询过程中如果用户没有账户信息，此时也要将用户信息查询出来，此时左外连接查询比较合适。

com.by.pojo下的Account类

package com.by.pojo;

import java.io.Serializable;
import java.util.Date;
import java.util.List;

public class User implements Serializable {
    private Integer id;
    private String username;
    private String password;
    private Date birthday;
    private String sex;
    private String address;
    //加入List<Account>存储用户所拥有的账户
    private List<Account> accounts;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public List<Account> getAccounts() {
        return accounts;
    }

    public void setAccounts(List<Account> accounts) {
        this.accounts = accounts;
    }

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

com.by.dao下的AcountDao接口

public interface AccountDao {
    List<Account> findAll();
}

com.by.dao下的UserDao.xml

<?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.by.dao.UserDao">
    <resultMap type="user" id="findAllResultMap">
        <id column="id" property="id"></id>
        <result column="username" property="username"/>
        <result column="address" property="address"/>
        <result column="sex" property="sex"/>
        <result column="birthday" property="birthday"/>
        <!-- collection 是用于建立一对多中集合属性的对应关系
        ofType 用于指定集合元素的数据类型
        -->
        <collection property="accounts" ofType="account">
            <id column="aid" property="id"/>
            <result column="uid" property="uid"/>
            <result column="money" property="money"/>
        </collection>
    </resultMap>
    <!-- 配置查询所有操作 -->
    <select id="findAll" resultMap="findAllResultMap">
      select u.*,a.id as aid ,a.uid,a.money 
      from user u left join account a on u.id =a.uid
    </select>
</mapper>

测试类

private SqlSession sqlSession;
    private InputStream inputStream;
    @Before
    public void init() throws IOException {
        //加载配置文件
        String resource = "mybatis-config.xml";
        inputStream = Resources.getResourceAsStream(resource);
        //创建SessionFactory
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //使用数据的会话实例
        sqlSession = sessionFactory.openSession();
    }
@After
    public void close() throws IOException {
        sqlSession.close();
        inputStream.close();
    }
@Test
    public void testOneToMany() {
        UserDao userDao = sqlSession.getMapper(UserDao.class);
        List<User> userList = userDao.findAll();
        for(User user : userList){
            System.out.println(user);
        }
    }

输出结果

三，多对多查询

需求：查询角色及角色赋予的用户信息。

分析：一个用户可以拥有多个角色，一个角色也可以赋予多个用户，用户和角色为双向的一对多关系，多对多关系其实我们看成是双向的一对多关系。

?com.by.pojo下的Role类

public class Role {
    private Integer id;
    private String roleName;
    private String roleDesc;
    //加入List<User> users存储角色赋予的用户信息
    private List<User> users;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getRoleName() {
        return roleName;
    }

    public void setRoleName(String roleName) {
        this.roleName = roleName;
    }

    public String getRoleDesc() {
        return roleDesc;
    }

    public void setRoleDesc(String roleDesc) {
        this.roleDesc = roleDesc;
    }

    public List<User> getUsers() {
        return users;
    }

    public void setUsers(List<User> users) {
        this.users = users;
    }

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

com.by.dao下的RoleDao接口

public interface RoleDao {
    List<Role> findAll();
}

com.by.dao下的RoleDao.xml

<?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.by.dao.RoleDao">
    <!--定义 role 表的 ResultMap-->
    <resultMap id="findAllResultMap" type="Role">
        <id property="id" 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="findAllResultMap">
        select r.id as rid,r.role_name,r.role_desc,u.* from role r
        left join user_role ur on r.id = ur.rid
        left join user u on u.id = ur.uid
    </select>
</mapper>

测试类

private SqlSession sqlSession;
    private InputStream inputStream;
    @Before
    public void init() throws IOException {
        //加载配置文件
        String resource = "mybatis-config.xml";
        inputStream = Resources.getResourceAsStream(resource);
        //创建SessionFactory
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        //使用数据的会话实例
        sqlSession = sessionFactory.openSession();
    }
@After
    public void close() throws IOException {
        sqlSession.close();
        inputStream.close();
    } 
 @Test
    public void testManyToMany() {
        RoleDao roleDao = sqlSession.getMapper(RoleDao.class);
        List<Role> roleList = roleDao.findAll();
        for(Role role : roleList){
            System.out.println(role);
        }
    }

测试结果