Question

我有以下两个表：

（我为在这里无法正确粘贴它们而道歉，因此创建了快照并给出了链接）：

表格考试

+---------+------------+-------------+  
| exam_id | exam_date  | description |  
+---------+------------+-------------+  
|       1 | 2016-06-01 | Exam 1      |  
|       2 | 2016-06-06 | Exam 2      |  
|       3 | 2016-06-07 | Exam 3      |  
|       4 | 2016-06-10 | Exam 4      |  
+---------+------------+-------------+

表科目

+------------+---------+-------------+  
| subject_id | exam_id | result      |  
+------------+---------+-------------+  
|          1 |       1 | Attended    |  
|          2 |       1 | Fail        |  
|          3 |       2 | Distinction |  
|          4 |       2 | Distinction |  
|          5 |       3 | Pass        |  
|          6 |       3 | Distinction |  
|          7 |       4 | Attended    |  
|          8 |       4 | Pass        |  
+------------+---------+-------------+

＆＃34;结果＆＃34;中的可能值领域是：

失败，出席，通过，区分

目的是根据该考试的下属科目取得的成绩来获得考试的总体结果 - ＆＃34;失败＆＃34;在该考试中的一个科目中，意味着总体结果是“失败”＃34; ＆＃34;出席＆＃34;意味着该主题的结果尚不确定，除非任何其他主题已经导致了＃34;失败＆＃34;等级，结果只能被称为＆＃34;参加＆＃34;。同样，＆＃34;区别＆＃34;只有在该考试的所有科目中取得成绩时才能达到。

查询：

SELECT exams.exam_id, exam_date, description, 
@count_fail         :=  SUM(result='Fail')          AS count_fail, 
@count_attended     :=  SUM(result='Attended')      AS count_attended, 
@count_pass         :=  SUM(result='Pass')          AS count_pass, 
@count_distinction  :=  SUM(result='Distinction')   AS count_distinction,
CASE 
    WHEN (@count_fail >= @count_attended AND @count_fail >= 1)
        THEN 'Fail'
    WHEN (@count_attended >= @count_pass AND @count_attended >= 1)
        THEN 'Attended'
    WHEN (@count_pass >= @count_distinction AND @count_pass>= 1)    
        THEN 'Pass'
    ELSE 
        'Distinction'
    END AS final_result FROM exams 
INNER JOIN subjects ON exams.exam_id = subjects.exam_id GROUP BY exam_id

结果令我难过......

+---------+------------+-------------+------------+----------------+------------+-------------------+--------------+
| exam_id | exam_date  | description | count_fail | count_attended | count_pass | count_distinction | final_result |
+---------+------------+-------------+------------+----------------+------------+-------------------+--------------+
|       1 | 2016-06-01 | Exam 1      |          1 |              1 |          0 |                 0 |  Distinction |
|       2 | 2016-06-06 | Exam 2      |          0 |              0 |          0 |                 2 |  Fail        |
|       3 | 2016-06-07 | Exam 3      |          0 |              0 |          1 |                 1 |  Distinction |
|       4 | 2016-06-10 | Exam 4      |          0 |              1 |          1 |                 0 |  Pass        |
+---------+------------+-------------+------------+----------------+------------+-------------------+--------------+

我可以问一下这里发生了什么？ WHEN条件是如何得不到的（很明显在@变量中进行了某种计算，因为输出不是所有的乱码）？这只是对更大的事物计划的一个小规模说明，遗憾的是我不能进入更精细的细节。但是，我真的想避免使用嵌套查询并尽可能多地使用JOINS ......

我非常感谢收到您的意见和建议。

非常感谢！

修改

数据库和查询的SQL小提琴是here

预期的输出是：

+---------+------------+-------------+------------+----------------+------------+-------------------+--------------+
| exam_id | exam_date  | description | count_fail | count_attended | count_pass | count_distinction | final_result |
+---------+------------+-------------+------------+----------------+------------+-------------------+--------------+
|       1 | 2016-06-01 | Exam 1      |          1 |              1 |          0 |                 0 |  Fail        |
|       2 | 2016-06-06 | Exam 2      |          0 |              0 |          0 |                 2 |  Distinction |
|       3 | 2016-06-07 | Exam 3      |          0 |              0 |          1 |                 1 |  Pass        |
|       4 | 2016-06-10 | Exam 4      |          0 |              1 |          1 |                 0 |  Attended    |
+---------+------------+-------------+------------+----------------+------------+-------------------+--------------+

理由是：

第1行：至少有一个科目达到了可能的最低等级（失败），因此总体结果必须是＆＃34;失败＆＃34;

第2行：该考试中的两个科目都已通过区分等级被清除，因此整体结果应该是＆＃34;区别＆＃34;

第3行： 1次传球，1次优异。这个结果不是一个区别，但是，它并不是一个失败的＆＃34;无论是。由于此考试中的两个科目都已被清除（虽然其中一个科目没有区分等级），结果应为＆＃34; Pass＆＃34;。

第4行： 1参加，1次通过。计数不足以得出结论总体结果将是＆＃34; Pass＆＃34; （因为＆＃34;参加＆＃34;是不确定的）。因此，总体结果仅符合“参加者”的标准。（直到考试中的所有科目都被＃34; Pass＆＃34;或者至少有一个＆＃34;失败＆＃34;清除，从而导致整个结果被调用＆＃ 34;失败＆＃34;。目前，＆＃34;区别＆＃34;现在无法实现，因为此考试中至少有一个科目已经注册了低级别成绩）

Answer 1

这里是solution as shown in SQLFiddle没有用户定义的变量

SELECT exams.exam_id, exam_date, description, 
SUM(result='Fail')          AS count_fail, 
SUM(result='Attended')      AS count_attended, 
SUM(result='Pass')          AS count_pass, 
SUM(result='Distinction')   AS count_distinction,
CASE 
    WHEN (SUM(result='Fail') >= SUM(result='Attended') AND SUM(result='Fail') >= 1)
        THEN 'Fail'
    WHEN (SUM(result='Attended') >= SUM(result='Pass')  AND SUM(result='Attended') >= 1)
        THEN 'Attended'
    WHEN (SUM(result='Pass')  >= SUM(result='Distinction') AND SUM(result='Pass') >= 1)    
        THEN 'Pass'
    ELSE 
        'Distinction'
    END AS final_result FROM exams 
INNER JOIN subjects ON exams.exam_id = subjects.exam_id
GROUP BY exam_id

MySQL - 在后续比较中使用派生列名称

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