Mysql查询时使用UNION和ORDER BY的问题

问题：
Mysql中表student_table(id,name,birth,sex)，查询男生、女生人数分别最多的3个姓氏及人数，正确的SQL是
A.

SELECT   sex ,substr(name,1,1) as first_name ,count(*) as c1 
from student_table where length(name) >=1  and sex = '男'
group by first_name  order by  sex ,c1 desc limit 3 
union all 
SELECT   sex ,substr(name,1,1) as first_name ,count(*) as c1 
from student_table where length(name) >=1  and sex = '女'
group by first_name  order by  sex ,c1 desc limit 3 ;

B.

select * from (
SELECT   sex ,substr(name,1,1) as first_name ,count(*) as c1 
from student_table where length(name) >=1  and sex = '男'
group by first_name  order by  sex ,c1 desc limit 3 
) t1 
UNION all
select * from (
SELECT   sex ,substr(name,1,1) as first_name ,count(*) as c1 
from student_table where length(name) >=1  and sex = '女'
group by first_name  order by  sex ,c1 desc limit 3 
) t2 ;

问题分析：
A和B选项从表面来看是一样的，A是B的简写。但是在UNION(ALL)中使用ORDER BY子查询时要注意UNION连接的语句只会出现一个ORDER BY（不包含子查询中的），否则会报sql未正确结束的错误。

解决措施：
将含ORDER BY的子查询包在一个不含ORDER BY的查询里再进行UNION ALL。