求一个简单的数据库查询SQL
数据库中表tb_A有3个字段：
ID int IDENTITY (1,1) PRIMARY KEY,
A INT NOT NULL,
B NVARCHAR (50)
问题是这样：
如果我需要查询出来tb_A中的A字段信息，按照ID进行倒序排列，A字段中有重复字段，要求去除重复。
请各位大侠帮忙看看该怎么写

------解决方案--------------------
--包含B 
select a.* from tb_a where id = (select min(id) from tb_a group by a) 

select a.* from tb_a a,
(select a,min(id) id from tb_a group by a) b
where a.id = b.id and a.a = b.a

select a.* from tb_a where id in (select min(id) from tb_a group by a)