Mybatis分步查询的实现示例

    <select id="listClassInfos" resultMap="listClassInfosMap">
        SELECT `id`, `name` FROM `class_info` WHERE NAME LIKE CONCAT('%', #{name}, '%') LIMIT #{pageStart}, #{pageSize}
    </select>
    <resultMap id="listClassInfosMap" type="com.example.study.vo.ClassInfoVo">
        <id column="id" property="id" jdbcType="INTEGER" javaType="java.lang.Integer"/>
        <result column="name" property="name" jdbcType="VARCHAR" javaType="java.lang.String"/>
        <!-- 分步查询1：select可以引用其它Mapper已有的查询语句，需要写全路径 -->
        <association property="master" column="{classId = id}"
                     select="com.example.study.mapper.ClassMasterInfoMapper.getMasterByClassId"/>
        <!-- 分步查询2：select也可以引用本Mapper的查询语句，不需要写全路径 -->
        <collection property="students" column="{classId = id}" select="listStudentsByClassId"/>
        <!-- 以上两个分步查询都可以添加fetchType="lazy"配置，在该查询中开启延迟加载，否则为全局默认 -->
    </resultMap>
    <select id="listStudentsByClassId" resultType="com.example.study.entity.StudentInfoEntity">
        SELECT
        `id` as `id`,
        `name` as `name`,
        `sex` as `sex`,
        `class_id` as `classId`
        FROM `student_info`
        WHERE `class_id` = #{classId}
    </select>
ClassMasterInfoMapper.xml中的查询：
    <select id="getMasterByClassId" resultMap="getMasterByClassIdMap">
        SELECT
        `id` as `id`,
        `class_id` as `classId`,
        `name` as `name`,
        `sex` as `sex`
        FROM `class_master_info`
        WHERE `class_id` = #{classId}
    </select>
    <resultMap id="getMasterByClassIdMap" type="com.example.study.entity.ClassMasterInfoEntity">
        <id column="id" property="id" jdbcType="INTEGER" javaType="java.lang.Integer"/>
        <result column="name" property="name" jdbcType="VARCHAR" javaType="java.lang.String"/>
        <result column="sex" property="sex" jdbcType="VARCHAR" javaType="java.lang.String"/>
        <!-- 分步查询1：select可以引用其它Mapper已有的查询语句，需要写全路径 -->
        <association property="self" column="{selfId = id}" select="self"/>
    </resultMap>
    <select id="self" resultType="com.example.study.entity.ClassMasterInfoEntity">
        SELECT
        `id` as `id`,
        `class_id` as `classId`,
        `name` as `name`,
        `sex` as `sex`
        FROM `class_master_info`
        WHERE `id` = #{selfId}
    </select>

# 第1个sql：查询最上层的数据，查到2条数据
SELECT `id`, `name` FROM `class_info` WHERE NAME LIKE CONCAT('%', ?, '%') LIMIT ?, ?
# 第2个sql：第1条数据，第1个子查询
SELECT
        `id` as `id`,
        `class_id` as `classId`,
        `name` as `name`,
        `sex` as `sex`
        FROM `class_master_info`
        WHERE `class_id` = ?
# 第3个sql：第1条数据，第1个子查询的第1个子查询（只有一个）
SELECT
        `id` as `id`,
        `class_id` as `classId`,
        `name` as `name`,
        `sex` as `sex`
        FROM `class_master_info`
        WHERE `id` = ?
# 第4个sql：第1条数据，第2个子查询
SELECT
        `id` as `id`,
        `name` as `name`,
        `sex` as `sex`,
        `class_id` as `classId`
        FROM `student_info`
        WHERE `class_id` = ?
# 第5个sql：第2条数据，第1个子查询
SELECT
        `id` as `id`,
        `class_id` as `classId`,
        `name` as `name`,
        `sex` as `sex`
        FROM `class_master_info`
        WHERE `class_id` = ?
# 第6个sql：第2条数据，第1个子查询的第1个子查询（只有一个）
SELECT
        `id` as `id`,
        `class_id` as `classId`,
        `name` as `name`,
        `sex` as `sex`
        FROM `class_master_info`
        WHERE `id` = ?
# 第7个sql：第2条数据，第2个子查询
SELECT
        `id` as `id`,
        `name` as `name`,
        `sex` as `sex`,
        `class_id` as `classId`
        FROM `student_info`
        WHERE `class_id` = ?