尝试在查询的where部分使用count列

Question

有2个名为

的表

学生

• stuID
• camID FK

校园

• camID PK
• camName

我想找到超过4名学生的校园，其中包括camName，camID，（学生人数）

这是我到目前为止所得到的

SELECT 
    students.camID, campus.camName, SUM(students.stuID) as [count] 
FROM 
    students 
JOIN 
    campus ON campus.camID = students.camID 
WHERE 
    [count] > 3 
GROUP BY 
    students.camID, campus.camName
ORDER BY 
    [count]

所有这一切都让我感到错误是“无效的comlumn名称'计数'。

Answer 1

您不能在WHERE子句中使用列别名，因为在甚至创建别名之前会对评估WHERE子句。您也不能在HAVING子句中使用别名。

SELECT students.camID, campus.camName, COUNT(students.stuID) as studentCount
FROM students
JOIN campus
    ON campus.camID = students.camID
GROUP BY students.camID, campus.camName
HAVING COUNT(students.stuID) > 3
ORDER BY studentCount

Answer 2

    SELECT [t0].* FROM campus AS [t0]
    INNER JOIN (SELECT COUNT(*) AS [value], [t1].camID
    FROM students AS [t1]
    GROUP BY [t1].camID ) 
    AS [t2] ON [t0].camID = [t2].camID
    WHERE [t2].[value] > 3

Answer 3

第一批SQL产品不支持派生表，因此发明了HAVING。但是现在我们确实有派生表，所以我们不再需要HAVING，实际上它可能会导致混淆（注意遗留功能永远不会从SQL标准中删除）：

SELECT * 
  FROM (
        SELECT students.camID, campus.camName,
               SUM(students.stuID) as [count]
          FROM students 
               JOIN campus ON campus.camID = students.camID 
         GROUP 
            BY students.camID, campus.camName
       ) AS DT1
 WHERE [count] > 3
 ORDER
    BY [count]