首先清楚：多个select 语句 union 时不是简单的将查询结果拼接起来 而是将sql拼接起来编译（做为一个sql语句），然后去执行。
注：
union 连接的语句中只会出现一个order by （不包含子查询中的）否则会报 sql未正确结束的错误。
解决方法：
将order by 语句放到子查询中
例子：
1、将排序后的查询结果拼接起来
select * from(select * from table order by a)
union
select * from (select * from table1 order by b)
union
select * from (select * from table2 order by c);
2、将排序后的查询结果拼接起来，再排序
select * from(select * from table order by a)
union
select * from (select * from table1 order by b)
union
select * from (select * from table2 order by c)
order by d;----此处的order by 是将最后的拼接结果集排序 打乱了之前通过 a、b、c的排序。