MySQL 中随机抽样(转)

MySQL中 随机抽取实现方法

1) 采用官方的方式
select * from tb_name order by rand() limit N

2)采用类似偏移方式
select * from tb_name
where id >= (select floor(max(id) * rand()) from tb_name )
order by id asc limit N;

3)优化的方式
select *
from tb_name as r1 join
(select round(rand() *
(select max(id)
from tb_name)) as id2)
as r2
where r1.id >= r2.id2
order by r1.id asc
limit N;

上述方法，第一种效率最低，第三种最好。