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union all
select * from t where cid in (3)
order by rand()
limit 1这样3的概率就是原来的一倍多
SELECT * FROM `t` WHERE `cid` IN (1,2,3)
group by `cid`
) v
ORDER BY rand() LIMIT 1
mysql> select * from ((select * from t1 where id=1 order by rand() limit 1) union (select * from t1
where id=2 order by rand() limit 1) union (select * from t1 where id=3 order by rand() limit 1)) b order by rand() limit 1;