正如您已经知道的，大多数人推荐的ORDER BY RAND()解决方案不能扩展到大型表。

SET @r := (SELECT FLOOR(RAND() * (SELECT COUNT(*) FROM mytable)));
SET @sql := CONCAT('SELECT * FROM mytable LIMIT 1 OFFSET ', @r);
PREPARE stmt1 FROM @sql;
EXECUTE stmt1;

我在我的书SQL Antipatterns Volume 1: Avoiding the Pitfalls of Database Programming中介绍了这个和其他解决方案。
如果你想用PHP做到这一点，你可以这样做（未测试）：

<?php
$mysqli->begin_transaction();
$result = $mysqli->query("SELECT COUNT(*) FROM mytable")
$row = $result->fetch_row(); 
$count = $row[0]; 
$offset = mt_rand(0, $count);
$result = $mysqli->query("SELECT * FROM mytable LIMIT 1 OFFSET $offset");
...
$mysqli->commit();

select ID, NAME, CLASS
from YOURTABLE
where CLASS='one'
order by rand()
limit $x

按rand()排序不是特别有效，但这是最小/最快的排序方式。

SELECT * FROM `table` WHERE `class`="one" ORDER BY RAND() LIMIT 5