请教一条SQL

有N条数据，每条数据有字段A，B，C，D，E，F
要取出数据X条
前Y条为按B排序的前Y条
接下来的Z条为剩下的数据按C排序的前Z条
最后的X－Y－Z条数据为前面剩下的数据按D排序的前X－Y－Z条

select * from (select * from item order by B desc limit 5)
union
select * from (select * from (select * from item order by B desc limit -1 offset 5) order by C desc limit 5)
union
select * from (select * from (select * from (select * from item order by B desc limit -1 offset 5) order by C desc limit -1 offset 5) order by D desc limit 5)
自己解决，但是有点长。。。