SQL NULL Values
處理 NULL
NULL 代表缺少值或未知值，通常用 IS NULL 或 IS NOT NULL 判斷。注意 NULL 不是空字串，也不是數值 0。
Example 1 - IS NULL
SELECT CustomerName, Address FROM Customers WHERE Address IS NULL;
Example 2 - IS NOT NULL
SELECT CustomerName, Address FROM Customers WHERE Address IS NOT NULL;
Example 3 - 找沒有配送日期的訂單
SELECT OrderID, ShippedDate FROM Orders WHERE ShippedDate IS NULL;
Example 4 - 用 COALESCE 補預設值
SELECT CustomerName,
COALESCE(Address, '未提供地址') AS AddressLabel
FROM Customers;