mysql随机查询符合条件的几条记录

select * from question q where q.`level`=1 order by rand() limit 1;

#随机查询（记录大于某个数，效率高）
select q1.* from question q1 inner join (select (min(q2.id) + round(rand()*(max(q2.id) - min(q2.id)))) as id from question q2 where q2.`level`=1) as t
on q1.id >= t.id limit 1;

#效率略低
select q.* from question q where q.id > (select t.id from (
    select (min(q2.id) + round(rand()*(max(q2.id) - min(q2.id)))) as id from question q2 where q2.`level`=1
) t) limit 1;

#效率极低，比order by rand还低（可能针对每条记录都作了子查询，结果不不连续，很随机）
select q.* from question q where q.id > (select (min(q2.id) + round(rand()*(max(q2.id) - min(q2.id)))) from question q2 where q2.`level`=1) limit 1;

select q1.* from question q1 inner join (select (min(q2.id) + round(rand()*(max(q2.id)-2 - min(q2.id)))) as id from question q2 where q2.`level`=1) as t
on q1.id >= t.id limit 3;

select q1.* from question q1 inner join (
    select (min(q2.id) + round(rand()* ( case when (max(q2.id)-2)>min(q2.id) then max(q2.id)-2 - min(q2.id) else 0 end ))) as id,min(q2.id) as minId,max(q2.id) as maxId from question q2 where q2.`level`=2
) as t
on q1.id >= t.id and q1.id between t.minId and t.maxId limit 3;

select 10*RAND(); #[0,10)
select FLOOR(10*RAND());#[0,9]
select CEILING(10*RAND()); #[1,10]
select ROUND(10*RAND()); #[0,10]