src/main/resources/mapper/PostMapper.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">

<mapper namespace="com.example.g8backend.mapper.PostMapper">
    <select id="getPostsByUserId" resultType="com.example.g8backend.entity.Post">
        SELECT * FROM posts WHERE user_id = #{userId}
    </select>
</mapper>