mysql使用union顺序混乱

使用mysql用union并子集中用order by排序顺序会混乱

1. (select id from a order by start_time asc)

     union all

    (select id from b order by start_time desc) limit 0, 20

这样的写法会导致排序混乱

解决方法如下

 

2.

 select * from

(select id from a order by start_time asc) t1

union all

select * from

(select id from b order by start_time desc) t2 limit 0, 20

 

这样就解决了 union子集需要再包一层才有作用