mysql - SQL Server - 返回 null 的 SELECT 语句

Question

下面的 SELECT 语句产生了这个结果

SELECT ProductID, SUM(Quantity) AS Quantity
FROM Production.ProductInventory
WHERE ProductID NOT IN (1001)
GROUP BY ProductID;

为什么下面编辑的 SELECT 语句没有产生任何结果？

SELECT ProductID, SUM(Quantity) AS Quantity
FROM Production.ProductInventory
WHERE Quantity >= 1800
GROUP BY ProductID;

Answer 1

您添加了 WHERE 子句:

SELECT ProductID, SUM(Quantity) AS Quantity
FROM Production.ProductInventory
WHERE Quantity >= 1800  -- no single row with Quantity that is higher than 1800
GROUP BY ProductID;

您可能想在聚合后过滤值:

SELECT ProductID, SUM(Quantity) AS Quantity
FROM Production.ProductInventory
GROUP BY ProductID
HAVING SUM(Quantity) >= 1800;   -- HAVING instead of WHERE

过滤:

• WHERE - 聚合前
• HAVING - 聚合后