mysql - IF 条件中 NULL 的计数

Question

我有这个查询:

    SELECT 

    SUM(IF(b.correct = 1,1,0)) correct_answers,
    SUM(IF(b.correct != 1,1,0)) incorrect_answers,
    SUM(IF(a.answer IS NULL) 1,0)) as no_answers,
    a.ID
    FROM `a` join b on a.answer  = b.id 
    where a.created_at between '2014-06-10' and '2014-06-17' 
    group by a.ID order by correct_answers desc

基本上，我试图获取每个用户的测验的正确答案、错误答案和无答案的数量。所以我得到了正确和错误的计数。但是，当我尝试计算没有答案的数量(表 a 中答案列为 NULL)时，我得到的全是零。这个查询有什么问题吗？

Answer 1

试试这个:

SELECT SUM(b.correct = 1) correct_answers,
       SUM(b.correct != 1) incorrect_answers,
       SUM(a.answer IS NULL) AS no_answers,
       a.ID
FROM a 
LEFT JOIN b ON a.answer = b.id 
WHERE a.created_at BETWEEN '2014-06-10' AND '2014-06-17' 
GROUP BY a.ID 
ORDER BY SUM(b.correct = 1) DESC;