SQL Syntax

大多數查詢可拆成 SELECT、FROM、WHERE、GROUP BY、HAVING、ORDER BY 這幾個區塊。掌握語句順序後，後面學習 JOIN、聚合與子查詢會順很多。

SELECT column1, column2
FROM table_name
WHERE condition
ORDER BY column1 ASC;

SELECT Country, COUNT(*) AS CustomerCount
FROM Customers
WHERE Country IS NOT NULL
GROUP BY Country
HAVING COUNT(*) >= 2
ORDER BY CustomerCount DESC;

SELECT c.CustomerName, o.OrderID
FROM Customers c
INNER JOIN Orders o ON c.CustomerID = o.CustomerID
WHERE o.OrderDate >= '2024-01-01';

SELECT ProductName,
       CASE
         WHEN Price >= 100 THEN 'Premium'
         ELSE 'Standard'
       END AS Tier
FROM Products;

注意： 實際資料庫不一定所有關鍵字都要寫，但常見查詢的整體順序通常固定。