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sql语句查询结果合并union 和union all用法_数据库技巧

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–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 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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