Union MYSQL中的GROUP BY

Question

我試圖讓每個教室的學生人數在場。

的條件是：

1. 從教室裏所有的學生其中ClassroomID是< 99和具有（DropTime <> '00：00' 和PickupTime = '00：00' ）和日期是20161111

2. 從教室裏所有的學生都在那裏ClassroomId> 99，並有（DropTime = '00：00' ）和日期是20161111

我使用日E採用在MySQL查詢並能正常工作，我得到正確的結果：

SELECT 
    COUNT(a.Id) AS total, c.Name 
FROM 
    Attendance a 
INNER JOIN 
    Classroom c ON a.classroom = c.Id 
WHERE 
    DropDate = '20161111' 
AND DropTime <> '00:00' 
AND PickupTime = '00:00' 
GROUP BY Name 

不同之處在於，當我嘗試做一個工會獲得另外一間教室的結果（即ClassroomID> 99），它並沒有帶來各教室，只帶1個教室。 這是我正在使用的聯合查詢;

SELECT 
SUM(total), Name 
FROM 
(SELECT 
    COUNT(a.Id) AS total, c.Name 
FROM 
    Attendance a 
INNER JOIN Classroom c ON a.classroom = c.Id 
WHERE 
    DropDate = '20161111' 
     AND DropTime <> '00:00' 
     AND PickupTime = '00:00' 
     AND c.Id < 99 
UNION ALL 
    SELECT 
    COUNT(a.Id) AS total, c.Name 
FROM 
    Attendance a 
INNER JOIN Classroom c ON a.classroom = c.Id 
WHERE 
    DropDate = '20161111' 
     AND PickupTime = '00:00' 
     AND c.Id > 99) t 
GROUP BY Name 

Answer 1

集團通過的必須到位針對每個聚集。因此你需要三個一組的; 1爲每個聯合聲明，因爲兩者都是聚合的，以及一個用於外部選擇...因爲你也在聚合。

換句話說，每個查詢必須能夠在它自己可以在另一個操作之前自己操作。你的兩個內聯查詢在大多數RDBMS系統中都會失敗，因爲缺少group by，但是MYSQL的擴展組使這個變複雜。

但是，可能有一種更有效的方式來編寫查詢......但這是一個基於您的初始嘗試的工作版本。

SELECT 
SUM(total), Name 
FROM 
(SELECT 
    COUNT(a.Id) AS total, c.Name 
FROM 
    Attendance a 
INNER JOIN Classroom c ON a.classroom = c.Id 
WHERE 
    DropDate = '20161111' 
     AND DropTime <> '00:00' 
     AND PickupTime = '00:00' 
     AND c.Id < 99 
GROUP BY c.name 
UNION ALL 
    SELECT 
    COUNT(a.Id) AS total, c.Name 
FROM 
    Attendance a 
INNER JOIN Classroom c ON a.classroom = c.Id 
WHERE 
    DropDate = '20161111' 
     AND PickupTime = '00:00' 
     AND c.Id > 99 
GROUP BY c.name) t 
GROUP BY t.name 

這似乎是simplier和應該達到相同的結果（只要我沒有螺絲的東西了），它應該是，因爲它沒有產生3個不同的組，然後將它們結合起來更快。

SELECT COUNT(a.Id) AS total 
    , c.Name 
FROM Attendance a 
INNER JOIN Classroom c 
    ON a.classroom = c.Id 
WHERE DropDate = '20161111' 
    AND ((DropTime <> '00:00' AND PickupTime = '00:00' AND c.Id < 99) 
    OR (PickupTime = '00:00' AND c.Id > 99)) 
GROUP BY c.name