src/main/resources/mapper/PostRatingMapper.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.mapper.PostRatingMapper">
    <insert id="insertOrUpdate">
        INSERT INTO post_ratings (user_id, post_id, rating)
        VALUES (#{userId}, #{postId}, #{rating})
        ON DUPLICATE KEY UPDATE rating = VALUES(rating)
    </insert>
</mapper>