当MySQL组中的条件不满足时显示计数0

Question

样本记录：

examid      setcode     answer
-------     --------   --------
10            A           A
10            A           B
10            A           X
10            B           A
10            B           B
10            B           C

我试图通过setcode找到计数组，答案是X.我尝试了以下查询：

SELECT setCode,COUNT(answer) FROM mcq_answer WHERE examid=10 AND answer='X' GROUP BY setCode

这个返回以下结果：

setcode     count
-------     -------- 
A            1  

但我正在寻找以下内容：

setcode     count
-------    -------- 
  A           1 
  B           0 

Setcode在这里是动态的。我只提到过A和B.可能有更多的setocdes如C，D，E，F等。我该怎么做。我正在使用MySQL

Answer 1

您可以使用SUM + IF来获得此结果：

SELECT `setCode`, SUM(if (`answer`='X', 1,0)) as answer 
FROM `mcq_answer`
WHERE examid=10
GROUP BY `setcode`

Answer 2

你可以使用以下查询

SELECT  C.setCode ,
    SUM(CASE WHEN B.setcode IS NULL THEN 0
             ELSE 1
        END) AS answer
FROM    ( SELECT    A.setCode ,
                COUNT(1) AS cnt
      FROM      mcq_answer A
      GROUP BY  A.setCode
    ) C
    LEFT JOIN mcq_answer b ON C.setcode = B.setcode
                              AND B.answer = 'X'
 GROUP BY  C.setCode

Answer 3

尝试：

SELECT A.setcode, 
COUNT(CASE WHEN B.answer ='X' THEN B.answer ELSE NULL END) as count_x
FROM
(SELECT DISTINCT setcode FROM YOUR_TABLE) A
LEFT JOIN
YOUR_TABLE B
ON A.setcode = B.setcode
GROUP BY A.setcode;

它也适用于setcode的动态值集。