Question

我想让每个上课的学生都在场。

条件是：

所有来自Classroom ID的教室的学生都是＆lt; 99和（DropTime＆lt;＆gt;＆＃39; 00＆＃39;和PickupTime =＆＃39; 00：00＆＃39;）和日期是20161111
所有来自ClassroomId的教室的学生都是＆gt; 99和（DropTime =＆＃39; 00：00＆＃39;）和日期是20161111

我在MySQL中使用以下查询，它工作正常，我得到了正确的结果：

 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

除了当我尝试联合以获得另一个教室的结果（即ClassroomID> 99）时，它不会带来每个教室，只带来1个教室的总数。这是我正在使用的联合查询;

 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

必须为每个聚合设置分组。因此，你需要三个小组;每个联合语句都有1个，因为它们都是聚合的，也是外部选择的一个......因为你也在那里聚合。

换句话说，每个查询必须能够在其可以在另一个查询中运行之前对其进行操作。由于缺少组，大多数RDBMS系统中的两个内联查询都会失败，但是MYSQL的扩展组会使这一点复杂化。

但是，可能有一种更有效的方式来编写查询......但这是一个基于您的初始尝试的工作版本。

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

这似乎更简单，并且应该达到相同的结果（假设我没有搞砸了）并且它应该更快，因为它不必生成3个不同的集合然后将它们组合起来。 / p>

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

Union MYSQL中的GROUP BY

1 个答案: