MySQL多结果集合并union查询

需求：
查询规则自定义，由数据库保存，返回结果字段来自同一张表；
要实现多个规则查询条件符合的适配，最终将查询结果合并展示出来，并标注是哪个规则的结果；
Mysql例子

--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  


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 
 

实践：
这里使用的是mysql默认的数据库mysql

SELECT * FROM (SELECT *,(SELECT 'A' as A) tag FROM help_topic) A
 UNION ALL
SELECT * FROM (SELECT *,(SELECT 'B' as A) tag FROM help_topic) B
 UNION ALL
SELECT * FROM (SELECT *,(SELECT 'C' as A) tag FROM help_topic) C
 UNION ALL
SELECT * FROM (SELECT *,(SELECT 'D' as A) tag FROM help_topic) D

效果图：