MySql 子查询排序问题

select * from a where a.id in(select id from b where b.name='xx' order by b.createdtime desc)


请问各位大侠 上面的语句 要按照select id from b where b.name='xx' order by b.createdtime desc这个的顺序输出
该怎么写？
在网上看到用 find_in_set 具体怎么用呢 怎么把select id from b where b.name='xx' order by b.createdtime desc这个弄到find_in_set 里呢？

select * from a 
inner join
(select id from b where b.name='xx' order by b.createdtime desc) b
on a.id=b.id
order by instr(b.id,a.id)