左连接返回零行

Question

我有两张桌子：

question
question_id | question

question_answer
answer_id | question_id | choice_id | user_id | explain

我想获得特定用户尚未回答的所有问题。

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

我在这种情况下得到零行。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。我坚持在这里如何进行此查询。

Answer 1

您检查特定用户的LEFT JOIN部分应该是相反的，即user_id = $userId而不是user_id <> $userId。

在结果行中，您可以选择user_id IS NULL来查找$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

Answer 2

如果您不一定要使用JOIN操作，那么您可以在

尝试此操作

  

想要获得特定用户未回答的所有问题   又：

select * from question
where
    question_id not in (select distinct question_id from question_answer where user_id=$userId);

See the demo on Sqlfiddle

不知道not in是否比JOIN操作更慢或更快。

Answer 3

请改为尝试：

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

SQL Fiddle Demo

这将为您提供 问题但没有答案 。

但是，如果您希望获得尚未回答问题的用户，则应该向users表额外添加JOIN。类似的东西：

SELECT u.id, u.name
FROM  users u 
LEFT JOIN question_answer qa ON qa.user_id = u.id
WHERE qa.question_id IS NULL;

Updated SQL Fiddle Demo

Answer 4

我会将上述MG的答案修改为

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=2
WHERE qa.user_id IS NULL;