src/main/resources/mapper/CategoryMybatisMapper.xml

<?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">
<!-- 修改1：使用全限定类名 -->
<mapper namespace="edu.bjtu.groupone.backend.mapper.CategoryMybatisMapper">

    <!-- 查询所有分类 -->
    <select id="findAll"
            resultType="edu.bjtu.groupone.backend.domain.entity.Category">
        SELECT * FROM categories
    </select>

    <!-- 根据父分类ID查询 -->
    <select id="findByParentId" parameterType="Long"
            resultType="edu.bjtu.groupone.backend.domain.entity.Category">
        SELECT * FROM categories WHERE parent_id = #{parentId}
    </select>

</mapper>