--合并重復行
select * from A
union
select * from B

--不合并重復行
select * from A
union all
select * from B

按某個字段排序
--合并重復行
select *
from (
select * from A
union
select * from B) AS T
order by 字段名

--不合并重復行
select *
from (
select * from A
union all
select * from B) AS T
order by 字段名

//sql server版
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
Select * From (
select top 2 id,adddate,title,url from bArticle where ClassId='3' order by adddate desc) C
Union All
Select * From (
select top 2 id,adddate,title,url from bArticle where ClassId='4' order by adddate desc) D

//MySQL版
Select * From (
select id,adddate,title,url from bArticle where ClassId='1' order by adddate desc limit 0,2) A
Union All
Select * From (
select id,adddate,title,url from bArticle where ClassId='2' order by adddate desc limit 0,2) B
Union All
Select * From (
select id,adddate,title,url from bArticle where ClassId='3' order by adddate desc limit 0,2) C
Union All
Select * From (
select id,adddate,title,url from bArticle where ClassId='4' order by adddate desc limit 0,2) D

在DB2里使用union時一定要注意相對應的字段一定是數據類型一樣，輸出時可以任意顯示的列，但不能輸出不同類型或不同表結構。因為這樣會出錯。