<resultMap id="getDetailsResultMap" type="StudentArchivesInfoExpressDetailsDTO">
    <id column="id" property="id"></id>
    <result column="student_id" property="studentId"></result>
    <collection property="express" columnPrefix="express_" ofType="ExpressDTO">
        <id column="id" property="id"></id>
        <result column="content" property="content"></result>
        <result column="time" property="time"></result>
    </collection>
</resultMap>

<select id="getDetails" resultMap="getDetailsResultMap">
    SELECT
        a.id,
        a.student_id,
        b.id AS express_id,
        b."content" AS express_content,
        b."time" AS express_time
    FROM
        t_student_archives_info_express AS a 
        LEFT JOIN t_express_message AS b
        ON a.id = b.relation_id AND b.relation_type = 1
</select>

//实体类
public class StudentArchivesInfoExpressDetailsDTO{
	private Long id;
	private Long studentId;
	private List<ExpressDTO> express;
}