SQL UNION
合併兩個查詢結果
UNION 會合併結果並去除重複列，UNION ALL 則保留全部列。兩個查詢的欄位數量與型別需要相容。
Example 1 - UNION 去重複
SELECT City FROM Customers UNION SELECT City FROM Suppliers ORDER BY City;
Example 2 - UNION ALL 保留全部列
SELECT City, 'Customer' AS SourceType FROM Customers UNION ALL SELECT City, 'Supplier' AS SourceType FROM Suppliers;