mysql - 左联接返回零行

我有两张桌子：

question
question_id | question

question_answer
answer_id | question_id | choice_id | user_id | explain

我想得到所有特定用户还没有回答的问题。

SELECT question, question_id as questionId
FROM question q
LEFT JOIN question_answer qa USING(question_id)
WHERE qa.user_id!=$userId

在这种情况下，我得到零行。我也试过了

SELECT question, q.question_id as questionId
FROM question q
LEFT JOIN question_answer qa ON q.question_id=qa.question_id AND qa.user_id!=$userId

显然这会返回所有记录。我一直在问这个问题。

最佳答案

检查特定用户的LEFT JOIN部分应该颠倒，即user_id = $userId而不是user_id <> $userId。
从结果行中选择user_id IS NULL的行，以查找$userId尚未回答的问题。

SELECT question, q.question_id as questionId
FROM question q
LEFT JOIN question_answer qa ON q.question_id=qa.question_id AND qa.user_id=$userId
WHERE qa.user_id IS NULL

关于mysql - 左联接返回零行，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/13399048/