select * from (
select *,
@group_index := CASE WHEN @prev_group = province THEN @group_index + 1 ELSE 1 END AS group_index,
@prev_group := province
from (
select * from sb_node order by province,rand()
) AS T1
) as T2 where group_index <=[n]
参考:http://stackoverflow.com/questions/2643314/mysql-group-by-limit
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