union与union all与order by

有时候我们会将两部分排序的结果联合起来显示，union会自动排序和去重,union all则不会

看以下sql语句：

select a,b,c from AA aa where ...... order by aa.a desc

union all

select a,b,c from BB bb where ...... order by bb.a desc

会报错，原因是union all 最后一个select才能orderby 或者就嵌套一层再order by,如下:

select * from (select a,b,c from AA aa where ...... order by aa.a desc)

union all

select * from (select a,b,c from BB bb where ...... order by bb.a desc)