MyBatis关联查询（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。
基础项目搭建参考第一个mybatis项目的创建和读取数据库信息
MyBatis关联查询（一、一对一查询）

在上面项目的基础上先编写user表的pojo文件

public class User implements Serializable {private Integer id;private String username;private Date birthday;private String sex;private String address;//多方private List<Account> accountList;public Integer getId() {return id;}public String getUsername() {return username;}public List<Account> getAccountList() {return accountList;}public void setAccountList(List<Account> accountList) {this.accountList = accountList;}public void setId(Integer id) {this.id = id;}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;}@Overridepublic String toString() {return "User{" +"id=" + id +", username='" + username + '\'' +", birthday=" + birthday +", sex='" + sex + '\'' +", address='" + address + '\'' +'}';}
}

编写UserMapper.java代码

public interface UserMapper {User getUserById(Integer id);}

在UserMapper.xml中编写下列代码

<resultMap id="getUserByIdResult" type="cn.fpl1116.pojo.User"><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"/><!--一堆多使用collection标签指定数据的封装规则--><collection property="accountList" ofType="cn.fpl1116.pojo.Account"><id column="aid" property="id"></id><result column="uid" property="uid"></result><result column="money" property="money"></result></collection></resultMap><select id="getUserById" parameterType="int" resultMap="getUserByIdResult">SELECT u.*, a.id aid, a.uid uid, a.money money FROM user u LEFT JOIN account a ON u.id=a.uid WHERE u.id=#{id}</select>

编写测试方法

@Testpublic void testGetUserById(){UserMapper userMapper = sqlSession.getMapper(UserMapper.class);User user = userMapper.getUserById(41);System.out.println(user);}