MySQL 在同一列上排序两次

Question

My database is like

id|service_id
1 | 8
2 | 3
3 | 4
4 | 1
5 | 3
6 | 2
7 | 1

i want to sort it like rows with same service_id came together but randomly. like

id|service_id
1 | 8
4 | 1
7 | 1
2 | 3
5 | 3
6 | 2
3 | 4

means first all rows sort via service_id then again sort via random service_id . I have tried SELECT * from sample_table order by service_id DESC and tried ASC too but it only do sort via DESC or ASC . I have tried order by rand(service_id) too but it is also showing some fixed sorting.

Answer 1

select t.*
from your_table t
join 
(
  select service_id, rand() as r
  from your_table 
  group by service_id
) tmp on t.service_id = tmp.service_id
order by tmp.r;

SQLFiddle demo

Basically this makes a random substitute for the service_id and orders by that