選擇最近一行並計算一對值的所有條目

Question

我必須顯示兩位候選人之間的會議列表，其中我必須顯示兩位候選人之間最近一次會議的日期和時間以及計數在他們之間安排的所有會議。

查詢我寫來選擇候選人之間的會議：這給

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 

以下ouptput：

+------------+--------------+--------------+------------------+-----------------+ 
| 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 | 
+------------+--------------+--------------+------------------+-----------------+ 

我們兩個之間的最近一次會議的細節相處計數應聘者，我將此添加到查詢中：

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; 

運行此程序後ERY我得到這個結果：

+------------+--------------+--------------+------------------+-----------------+---------------+ 
| 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 | 
+------------+--------------+--------------+------------------+-----------------+---------------+ 

但該預期的結果

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

PS：請忽略預期的結果id列的順序。

Answer 1

實際輸出和預期輸出之間的差異的原因是，你沒有告訴MySQL你想要最近的會議日期。您的查詢是針對sql標準的，因爲您在選擇列表中既沒有列出分組，也沒有聚合函數的主題（如max()）。

MySQL在某些sql模式設置下允許這樣的查詢運行，但是value of such fields is indeterminate。

最好的解決辦法是明確地告訴MySQL你需要什麼：

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 

如果你想從最新的會議記錄candidate_meetings表附加字段的值，然後將上面的查詢在子查詢中，並加入它返回到您的candidate_meetings表中concat(candidate_meetings.meeting_date, candidate_meetings.meeting_time)=subquery_alias.latest_meeting_time連接條件的外部查詢。

Answer 2

試試這個：

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 

你得到預期的結果錯了，因爲當你使用GROUP BY子句，數據庫選擇MEETING_ID，meeting_date，meeting_time最後一次出現。