union all中使用order by子查询时需将含order by的子查询包在一个不含order by的查询里再进行union all。
否则会报“Incorrect usage of UNION and ORDER BY”
例如：
1.错误用法：
select id from test1 order by id
union all
select id from test2 order by id
2.正确用法：
select * from (select id from test1 order by id) t1
union all
select * from (select id from test2 order by id) t2