SQL UNION 與 UNION ALL 用法詳解：合併查詢結果與排序技巧

UNION 合併結果並去除重複列；UNION ALL 保留所有列（含重複），效能較佳。

基本語法

-- 去重合併
SELECT * FROM A UNION SELECT * FROM B;

-- 保留重複
SELECT * FROM A UNION ALL SELECT * FROM B;

合併後排序

需包裹子查詢再 ORDER BY：

SELECT * FROM (SELECT * FROM A UNION ALL SELECT * FROM B) AS T ORDER BY 欄位名;

分頁取各類別最新 N 筆（SQL Server / MySQL 寫法對照）

-- SQL Server (TOP)
SELECT * FROM (SELECT TOP 2 id,adddate,title,url FROM bArticle WHERE ClassId=1 ORDER BY adddate DESC) A
UNION ALL
SELECT * FROM (SELECT TOP 2 id,adddate,title,url FROM bArticle WHERE ClassId=2 ORDER BY adddate DESC) B
UNION ALL ...;

-- MySQL (LIMIT)
SELECT * FROM (SELECT id,adddate,title,url FROM bArticle WHERE ClassId=1 ORDER BY adddate DESC LIMIT 0,2) A
UNION ALL ...;