Mybatis 一对多关联查询的两种方式：嵌套结果与嵌套查询

嵌套结果

Mapper 接口

List<TUser> selectUserJobs1();

返回结果会将查出来的表根据<id>合并

Mapper XML配置

<resultMap id="BaseResultMap" type="TUser">	
	<id column="id" property="id" />
		<result column="userName" property="userName" />
	<result column="realName" property="realName" />
	<result column="sex" property="sex" />
	<result column="mobile" property="mobile" />
	<result column="email" property="email" />
	<result column="note" property="note" />
</resultMap>
<resultMap id="userAndJobs1" extends="BaseResultMap" type="TUser">
	<collection property="jobs"
		ofType="com.enjoylearning.mybatis.entity.TJobHistory" >
		<result column="comp_name" property="compName" jdbcType="VARCHAR" />
		<result column="years" property="years" jdbcType="INTEGER" />
		<result column="title" property="title" jdbcType="VARCHAR" />
	</collection>
</resultMap>
<select id="selectUserJobs1" resultMap="userAndJobs1">
	select
	a.id,
	a.userName,
	a.realName,
	a.sex,
	a.mobile,
	b.comp_name,
	b.years,
	b.title
	from t_user a,
	t_job_history b
	where a.id = b.user_id
</select>

TUser 对象

@Data
public class TUser{
	
    private Integer id;

    private String userName;

    private String realName;

    private Byte sex;

    private String mobile;
	
    private String email;

    private String note;
    //一对多的体现
    private List<TJobHistory> jobs ;
}

嵌套查询

Mapper 接口

List<TUser> selectUserJobs2();

Mapper XML配置

<resultMap id="userAndJobs2" extends="BaseResultMap" type="TUser">
	<collection property="jobs" fetchType="lazy" column="id"
		select="com.enjoylearning.mybatis.mapper.TJobHistoryMapper.selectByUserId" />
</resultMap>
<select id="selectUserJobs2" resultMap="userAndJobs2">
	select
	a.id,
	a.userName,
	a.realName,
	a.sex,
	a.mobile
	from t_user a
</select>

这种方式会把所有的 TUser 查出来，t_user 有多少条，就会查出多少条