Question

我必须显示两名候选人之间的会议清单，其中我必须显示两名候选人之间最近一次会议的日期，时间以及他们之间安排的所有会议的次数。

查询我已写信选择候选人之间的会议：

counts = 1

给出以下输出：

SELECT cm.id AS meeting_id, cm.meeting_date, cm.meeting_time, cm.candidate1_id, cm.candidate2_id
            FROM candidate_meetings AS cm                                   
            JOIN view_candidate_details AS candidate_1 ON candidate_1.id = cm.candidate1_candidate_id
            JOIN view_candidate_details AS candidate_2 ON candidate_2.id = cm.candidate2_candidate_id
            WHERE (candidate_1.counselor_member_id = 41 OR candidate_2.counselor_member_id = 41)
            ORDER BY cm.id DESC

现在要计算两个候选人之间最近一次会议的详细情况，我将其添加到查询中：

+------------+--------------+--------------+------------------+-----------------+
| meeting_id | meeting_date | meeting_time | candidate1_id    | candidate2_id   |
+------------+--------------+--------------+------------------+-----------------+
|          6 | 2016-08-31   | 17:45:00     |               24 |              28 |
|          5 | 2016-08-31   | 17:30:00     |               24 |              28 |
|          4 | 2016-08-31   | 19:30:00     |               24 |              23 |
|          3 | 2016-08-31   | 18:30:00     |               24 |              22 |
|          2 | 2016-08-31   | 19:15:00     |               24 |              21 |
|          1 | 2016-08-31   | 17:15:00     |               24 |              21 |
+------------+--------------+--------------+------------------+-----------------+

运行此查询后，我得到了这个结果：

SELECT *, count(*) AS meeting_count                                
        FROM                                                                
        (                                                                   
            SELECT cm.id AS meeting_id, cm.meeting_date, cm.meeting_time, cm.candidate1_id, cm.candidate2_id
            FROM candidate_meetings AS cm                                   
            JOIN view_candidate_details AS candidate_1 ON candidate_1.id = cm.candidate1_candidate_id
            JOIN view_candidate_details AS candidate_2 ON candidate_2.id = cm.candidate2_candidate_id
            WHERE (candidate_1.counselor_member_id = 41 OR candidate_2.counselor_member_id = 41)
            ORDER BY cm.id DESC             
        ) as sub                                                            
        GROUP BY sub.candidate1_id, sub.candidate2_id;

但预期的结果是

+------------+--------------+--------------+------------------+-----------------+---------------+
| meeting_id | meeting_date | meeting_time | candidate1_id    | candidate2_id   | meeting_count |
+------------+--------------+--------------+------------------+-----------------+---------------+
|          1 | 2016-08-31   | 17:15:00     |               24 |              21 |             2 |
|          3 | 2016-08-31   | 18:30:00     |               24 |              22 |             1 |
|          4 | 2016-08-31   | 19:30:00     |               24 |              23 |             1 |
|          5 | 2016-08-31   | 17:30:00     |               24 |              28 |             2 |
+------------+--------------+--------------+------------------+-----------------+---------------+

PS：请忽略预期结果中id列的顺序。

Answer 1

实际输出与预期输出之间存在差异的原因是您没有告诉MySQL您想要最新的会议日期。您的查询是针对sql标准的，因为您在选择列表中的字段既不在分组列表中，也不是max()等聚合函数的主题。

MySQL在某些sql模式设置下允许运行此类查询，但value of such fields is indeterminate。

最好的解决方案是明确告诉MySQL你需要什么：

SELECT  max(concat(cm.meeting_date, cm.meeting_time)) as latest_meeting_time, cm.candidate1_id, cm.candidate2_id
        FROM candidate_meetings AS cm                                   
        JOIN view_candidate_details AS candidate_1 ON candidate_1.id = cm.candidate1_candidate_id
        JOIN view_candidate_details AS candidate_2 ON candidate_2.id = cm.candidate2_candidate_id
        WHERE (candidate_1.counselor_member_id = 41 OR candidate_2.counselor_member_id = 41)
        GROUP BY cm.candidate1_id, cm.candidate2_id

如果您想要来自最新会议的candidate_meetings表中的其他字段值，请访问：记录，然后将上述查询放在子查询中，并将其连接到candidate_meetings连接条件的外部查询中的concat(candidate_meetings.meeting_date, candidate_meetings.meeting_time)=subquery_alias.latest_meeting_time表。

Answer 2

试试这个：

SELECT *, count(*) AS meeting_count                                
FROM                                                                    
    (SELECT cm.id AS meeting_id, cm.meeting_date, cm.meeting_time, cm.candidate1_id, cm.candidate2_id
    FROM candidate_meetings AS cm                                   
    JOIN view_candidate_details AS candidate_1 ON candidate_1.id = cm.candidate1_candidate_id
    JOIN view_candidate_details AS candidate_2 ON candidate_2.id = cm.candidate2_candidate_id
    WHERE (candidate_1.counselor_member_id = 41 OR candidate_2.counselor_member_id = 41)
    ORDER BY cm.id             
) as sub                                                            
GROUP BY sub.candidate1_id, sub.candidate2_id
ORDER BY cm.id DESC

您得到的预期结果是错误的，因为当您使用GROUP BY子句时，数据库会选择最后一次出现的meeting_id，meeting_date，meeting_time。

选择最近一行，其中包含一对值的所有条目的计数

2 个答案: