sql 查询结果合并union all用法_数据库技巧

--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 2 id,adddate,title,url from bArticle where ClassId=1 order by adddate desc) A
Union All
Select From (
select 2 id,adddate,title,url from bArticle where ClassId=2 order by adddate desc) B
Union All
Select From (
select 2 id,adddate,title,url from bArticle where ClassId=3 order by adddate desc) C
Union All
Select From (
select 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