如何使用mybatis进行多表联查？

先看如下链接博客，再来观看本篇文章，两者对比，可能会有不一样的感受。
                        
原文链接：https://blog.youkuaiyun.com/2301_79965602/article/details/136605149

实现：根据套餐id查询套餐详情接口

在套餐详情页面需要展示当前套餐的信息（包括图片、套餐名称、套餐介绍、适用性别、适用年龄）、此套餐包含的检查组信息、检查组包含的检查项信息等。

此时就涉及到了多张表，我们就可以利用mybatis的多表联查来解决！

1、首先，根据套餐id查询套餐

<?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.itheima.dao.UserSetmealDao">

    <resultMap id="baseResultMap" type="com.itheima.pojo.Setmeal">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="code" property="code"/>
        <result column="helpCode" property="helpCode"/>
        <result column="sex" property="sex"/>
        <result column="age" property="age"/>
        <result column="price" property="price"/>
        <result column="remark" property="remark"/>
        <result column="attention" property="attention"/>
        <result column="img" property="img"/>
    </resultMap>

    <resultMap id="findByIdResultMap" type="com.itheima.pojo.Setmeal" extends="baseResultMap">
        <!--套餐和检查组多对多映射-->
        <collection
                property="checkGroups"
                ofType="com.itheima.pojo.CheckGroup"
                select="com.itheima.dao.UserCheckGroupDao.findCheckGroupById"
                column="id">
        </collection>
    </resultMap>


    <!--根据套餐id查询套餐详情-->
    <select id="findById" parameterType="int" resultMap="findByIdResultMap">
        SELECT * FROM t_setmeal WHERE id = #{id}
    </select>
</mapper>

2、然后再根据套餐id查询关联的检查组信息（注意关联的名称不要弄错）

<?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.itheima.dao.UserCheckGroupDao">
    <resultMap id="baseResultMap" type="com.itheima.pojo.CheckGroup">
        <id column="id" property="id"/>
        <result column="code" property="code"/>
        <result column="name" property="name"/>
        <result column="helpCode" property="helpCode"/>
        <result column="sex" property="sex"/>
        <result column="remark" property="remark"/>
        <result column="attention" property="attention"/>
    </resultMap>

    <resultMap id="findByIdResultMap" type="com.itheima.pojo.CheckGroup" extends="baseResultMap">
        <!--检查项和检查组多对多映射-->
        <collection property="checkItems"
                    ofType="com.itheima.pojo.CheckItem"
                    column="id"
                    select="com.itheima.dao.UserCheckItemDao.findCheckItemById">
        </collection>
    </resultMap>

    <!--根据套餐id查询关联的检查组详情-->
    <select id="findCheckGroupById" parameterType="int" resultMap="findByIdResultMap">
        SELECT * FROM t_checkgroup WHERE id IN (SELECT checkgroup_id FROM t_setmeal_checkgroup WHERE setmeal_id = #{id})
    </select>
</mapper>

3、最后，根据检查组id查询关联的检查项详情

<?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.itheima.dao.UserCheckItemDao">
    <!--根据检查组id查询关联的检查项详情-->
    <select id="findCheckItemById" parameterType="int" resultType="com.itheima.pojo.CheckItem">
        SELECT * FROM t_checkitem WHERE id IN (SELECT checkitem_id FROM t_checkgroup_checkitem WHERE checkgroup_id = #{id})
    </select>
</mapper>

就这三步，就搞定啦！