SQL缺少记录问题

Question

大家好，我很抱歉这个愚蠢的问题，但我遇到了一个问题，希望你们能指点我 - 正确的方向请耐心等待我是一个完整的初学者。 我有三个表的设置，看起来大致如下：

table 1 quizquestions 
Question ID, etc. 
Int 

table 2 quizanswers 
AnswerID, QuestionID,correct Answer 
Int   Int   Bool 

table 3 useranswers 
UserAnswerID, AnswerID, QuestionID 
Int    Int   Int 

我的问题是，当我运行查询像这样：

SELECT A.AnswerID AS Answer, UA.AnswerID AS UserAnswer 
FROM quizanswers AS A, useranswers UA 
WHERE A.CorrectAnswer = TRUE 
    AND UA.QuestionID 
    AND A.QuestionID = X 

查询将返回一个结果类似如下：

Answer UserAnswer 
    1  1 

如果在quizanswers和useranswers表中都有记录，则查询将成功返回结果。然而，问题在于useranswers表中没有记录时，不会返回任何结果。如果上面的代码不够清楚，我很抱歉，很多预先感谢。

Answer 1

我不知道，我得到这个问题背后的商业案例，但是这会给你quizanswers记录即使在useranswers没有correspondning记录：

SELECT A.AnswerID AS Answer, UA.AnswerID AS UserAnswer FROM quizanswers AS A LEFT JOIN useranswers UA ON UA.QuestionID = A.QuestionD WHERE A.CorrectAnswer = TRUE AND A.QuestionID = X 

Answer 2

这将在MSSQL中工作我相信我没有MySQL的正确的空函数，但逻辑应该是相同的。

SELECT A.AnswerID AS Answer, UA.AnswerID AS UserAnswer 
FROM quizanswers AS A LEFT OUTER JOIN useranswers UA 
ON UA.QuestionID = A.QuestionID 
WHERE ISNULL(A.CorrectAnswer, FALSE) = TRUE