因为mysql数据的最大值和最小值的获取几乎是0消耗的，数据库表数据与自身主键ID的集合做join查询，是非常快速的。
1：select * from users order by rand() LIMIT 10000
在ORDER BY从句里面不能使用RAND()函数，
因为这样会导致数据列被多次扫描，导致效率相当相当的低！效率不行，切忌使用！
2：SELECT * FROM users AS t1 JOIN
(
SELECT ROUND
(RAND() * ((SELECT MAX(userId) FROM `users`)-(SELECT MIN(userId) FROM users))+(SELECT MIN(userId) FROM users))
AS userId
)
AS t2
WHERE t1.userId >= t2.userId
ORDER BY t1.userId
LIMIT 10000
ROUND()四舍五入，0<=RAND()<1 随机数
3：SELECT * FROM users WHERE userId >= ((SELECT MAX(userId) FROM users)-(SELECT MIN(userId) FROM users)) * RAND() + (SELECT MIN(userId) FROM users) LIMIT 10000