SQL：在同一select語句中使用函數中列的別名

Question

我只是想簡化一個Select語句：

Select (a+b+c)/3 AS V, MIN((a+b+c)/3) as Min, MAX((a+b+c)/3) as Max from ....

是否有一種干凈的方法可以避免在聚合函數中重復該公式（a + b + c）/ 3？

Answer 1

SELECT 
  Result/3 AS V, 
  MIN(Result/3) as Min, 
  MAX(Result/3) as Max
FROM
(
  SELECT (a+b+c) AS Result from Table
) AS outerTable

甚至

SELECT 
  Result AS V, 
  MIN(Result) as Min, 
  MAX(Result) as Max
FROM
(
  SELECT (a+b+c)/3 AS Result from Table
) AS outerTable

Answer 2

如果您需要的值不僅僅是一個查詢中的值(a+b+c)/3而且您的SQL方言支持視圖，那么您可能需要考慮使用View。

可以在MySQL中使用以下代碼創建一個View，其中包含表MY_TABLE_NAME所有列+包含值(a+b+c)/3的附加列。

CREATE ALGORITHM = UNDEFINED
SQL SECURITY DEFINER VIEW  `MY_TABLE_NAME_FULL_VIEW` AS
SELECT MY_TABLE_NAME.*, (a+b+c)/3 AS Average
   FROM  `MY_TABLE_NAME`;

可以在MySQL中使用以下代碼創建一個僅包含a，b和c列的視圖以及包含值(a+b+c)/3的附加列。

CREATE ALGORITHM = UNDEFINED
SQL SECURITY DEFINER VIEW  `MY_TABLE_NAME_LIMITED_VIEW` AS
SELECT a, b, c, (a+b+c)/3 AS Average
   FROM  `MY_TABLE_NAME`;

可以在MySQL中使用以下代碼在MySQL中創建僅包含值(a+b+c)/3的View。

CREATE ALGORITHM = UNDEFINED
SQL SECURITY DEFINER VIEW  `MY_TABLE_NAME_AVERAGE_VIEW` AS
SELECT (a+b+c)/3 AS Average
   FROM  `MY_TABLE_NAME`;

創建視圖后，在選擇查詢中使用視圖而不是表。

---

假設您創建了視圖MY_TABLE_NAME_AVERAGE_VIEW ，您可以將SELECT語句替換為：

SELECT Average AS V, MIN(Average) AS Min, MAX(Average) AS Max
FROM MY_TABLE_NAME_AVERAGE_VIEW;

假設您創建了視圖MY_TABLE_NAME_LIMITED_VIEW ，並且還需要包含值a，b和C，則可以將SELECT語句替換為：

SELECT a, b, c, Average AS V, MIN(Average) AS Min, MAX(Average) AS Max
FROM MY_TABLE_NAME_LIMITED_VIEW;