求一MySql查询语句,该如何解决

求一MySql查询语句

SQL code

  
table a:
aid na nb
0   1  2
1   1  3
2   3  2

查询结果：
nab
1
2
3

结果为字段na和字段nb的记录去除重复后的汇总。

SQL code

select nab
from (
select na as nab from a
union
select nb as nab fromm a
)T
order by T.nab


错了

  
  

   
   SQL code
   
   
select nab
from (
select distinct na as nab from a
union
select distinct nb as nab fromm a
)T
order by T.na


没有其它好方法了。