我需要这一个这样的select 语句

**我需要这样的select 语句 **

select max_B from (select A, max(B) max_B from xxxtable group by A having count(A)>1) ;

select a,max(b) from tablename group by a

select 字段A,字段B from table_t
order by 字段A desc,字段B desc

select * from (select * from 表明 order by 字段B desc）as table group by 字段A