mysql sql 语句调优 rand()为啥不稳定

请教一下 第二个sql 为啥 会查出null 数据， 或者出现两条数据 我把sql 贴出来

    SELECT a.id,a.pid 
    FROM `shopinfo` a  
    WHERE a.id=(select id 
                            from ( SELECT id,rand() as random_num 
                                         FROM bookinfo 
                                         where status='1') a 
                            order by a.random_num 
                            limit 1)      




            SELECT a.id,a.pid
            FROM `shopinfo` a  
            WHERE a.id=( SELECT id 
                                    FROM bookinfo 
                                    where status='1'  
                                    order by rand() 
                                    LIMIT 1 )

SELECT a.id,a.pid
FROM shopinfo a

WHERE a.id=(select id
from ( SELECT id,rand() as random_num
FROM bookinfo
where status='1') a
order by a.random_num
limit 1)
最外层的where 应该不是a.id吧 a.id与子查询的id是不同表的id 那当然可能有数据 可能没数据了

SELECT a.id,a.pid
FROM shopinfo a

WHERE a.id=(select id
from ( SELECT id,rand() as random_num
FROM bookinfo
where status='1') a
order by a.random_num
limit 1)
最外层的where 应该不是a.id吧 a.id与子查询的id是不同表的id 那当然可能有数据 可能没数据了

因为bookinfo里面随机取的id在shopinfo里不存在？
建议两张表直联之后再rand查询，就不会null了，而且子查询效率也低啊。。
select a.id, a.pid from shopinfo a, bookinfo b where a.id=b.id and b.status='1' order by rand() limit 1

rand本身确实是不稳定的，你要将它设置为基于时间的rand，这样就可以了。