sql话语查询结果合并union 和union all用法

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sql语句查询结果合并union 和union all用法

--合并重复行
select * from A
union
select * from B

--不合并重复行
select * from A
union all
select * from B

按某个字段排序
--合并重复行
select *
from (
select * from A
union
select * from B) AS T
order by 字段名

--不合并重复行
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 byadddate desc) A
Union All
Select * From (
select top 2 id,adddate,title,url from bArticle where ClassId='2' order byadddate desc) B
Union All
Select * From (
select top 2 id,adddate,title,url from bArticle where ClassId='3' order byadddate desc) C
Union All
Select * From (
select top 2 id,adddate,title,url from bArticle where ClassId='4' order byadddate desc) D

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

在DB2里使用union时一定要注意相对应的字段一定是数据类型一样，输出时可以任意显示的列，但不能输出不同类型或不同表结构。因为这样会出错。

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