SQL GROUP BY

分組統計

GROUP BY 用來依欄位分組，幾乎總是與聚合函式一起出現。它是做國家別、月份別、產品別分析的基礎。

Example 1 - 依國家統計客戶數

SELECT Country, COUNT(*) AS CustomerCount
FROM Customers
GROUP BY Country
ORDER BY CustomerCount DESC;

Example 2 - 依客戶統計訂單數

SELECT CustomerID, COUNT(*) AS OrderCount
FROM Orders
GROUP BY CustomerID
ORDER BY OrderCount DESC;

Example 3 - 多欄位分組

SELECT ShipCountry, YEAR(OrderDate) AS OrderYear, COUNT(*) AS TotalOrders
FROM Orders
GROUP BY ShipCountry, YEAR(OrderDate)
ORDER BY ShipCountry, OrderYear;