mysql使用union順序混亂

使用mysql用union並子集中用order by排序順序會混亂

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

     union all

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

這樣的寫法會導致排序混亂

解決方法如下

 

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

 

這樣就解決了 union子集需要再包一層才有作用