sql - LEFT JOIN 返回空结果集

Question

我在连接方面遇到了一个小逻辑问题。我有一个用于回答问题的数据库。架构是:

Question
question_id
question_text

Answers
answer_id
question_id
answer_text

User Responses
user_id
answer_id
question_id

我正在尝试查找用户尚未回答的问题，但我一直收到空回复。查询如下:

SELECT * FROM questions 
    LEFT JOIN responses ON questions.question_id = responses.question_id 
WHERE user_id != '1'

我的逻辑哪里出了问题？

Answer 1

尝试使用 IS NULL 进行 LEFT JOIN

SELECT q.question_id FROM questions q
    LEFT JOIN responses r ON q.question_id = r.question_id AND r.user_id = 1
WHERE r.question_id IS NULL