SQL Join包含NULL值

Question

我正在尝试将3个表连接到一个select语句中，但如果没有出现，则计算一个表的出现，同时仍显示记录。

我的例子可以在我放在一起的快速sqlFiddle中看到。我试图使用左连接，但它不会产生我想要的结果。

http://sqlfiddle.com/#!6/e2840/8

这是SQL语句：

SELECT O.OptionID,O.OptionName, Count(A.OptionID) AS Total
FROM Options as O
    LEFT JOIN Answers AS A  ON O.OptionID = A.OptionID 
    LEFT JOIN Users as U ON A.UserId = U.UserID  
WHERE   A.QuestionID = 1
GROUP BY O.OptionID,O.OptionName

我希望它返回的是选项表中的所有行，如果没有找到该选项的答案，则在总列中显示0。

目前的小提琴回归：

我希望它返回：

Answer 1

您正在使用带有LEFT JOIN的WHERE子句，这使其像内部联接一样。

试试这个：

SELECT O.OptionID, O.OptionName, Count(A.OptionID) AS Total
FROM Options as O
    LEFT JOIN Answers AS A  ON O.OptionID = A.OptionID 
    and A.QuestionID = 1
    LEFT JOIN Users as U ON A.UserId = U.UserID  
GROUP BY O.OptionID,O.OptionName

Answer 2

选择答案：

SELECT O.OptionID,O.OptionName, Count(A.OptionID) AS Total
FROM Options as O
LEFT JOIN 
(
    SELECT OPTIONID,USERID 
    FROM Answers WHERE QuestionID = 1
) AS A  ON O.OptionID = A.OptionID 
LEFT JOIN Users as U ON A.UserId = U.UserID  
GROUP BY O.OptionID,O.OptionName

Answer 3

尝试下面的一些事情......

       SELECT O.OptionID,O.OptionName, Count(A.OptionID) AS Total
       FROM Options as O
       LEFT JOIN Answers AS A  ON O.OptionID = A.OptionID and A.questionId=1
       LEFT JOIN Users as U ON A.UserId = U.UserID  
       GROUP BY O.OptionID,O.OptionName

Answer 4

未引用Users表，并且这与使用sqlfiddle中的数据匹配预期结果。

SELECT O.OptionID,O.OptionName, Count(A.OptionID) AS Total
FROM Options as O
    LEFT JOIN Answers AS A  ON O.OptionID = A.OptionID
                            AND A.QuestionID = 1
GROUP BY O.OptionID,O.OptionName

顺便说一下：使用子查询没有任何好处。添加的连接条件完成相同的工作。

但是，通过避免冗余连接，您将获得优势。

Answer 5

与@mjsqu相同但会避免子集。这是加入子集的更好方法

SELECT O.OptionID,O.OptionName, Count(A.OptionID) AS Total
FROM Options as O
    LEFT JOIN Answers AS A  ON O.OptionID = A.OptionID and A.QuestionID = 1
    LEFT JOIN Users as U ON A.UserId = U.UserID  
GROUP BY O.OptionID,O.OptionName