我需要一个查询从一个相当大的表中返回大量随机行。这是我想到的。首先获取最大记录id:

SELECT MAX(id) FROM table_name;

然后将该值代入:

SELECT * FROM table_name WHERE id > FLOOR(RAND() * max) LIMIT n;

Where max is the maximum record id in the table and n is the number of rows you want in your result set. The assumption is that there are no gaps in the record id's although I doubt it would affect the result if there were (haven't tried it though). I also created this stored procedure to be more generic; pass in the table name and number of rows to be returned. I'm running MySQL 5.5.38 on Windows 2008, 32GB, dual 3GHz E5450, and on a table with 17,361,264 rows it's fairly consistent at ~.03 sec / ~11 sec to return 1,000,000 rows. (times are from MySQL Workbench 6.1; you could also use CEIL instead of FLOOR in the 2nd select statement depending on your preference)

DELIMITER $$

USE [schema name] $$

DROP PROCEDURE IF EXISTS `random_rows` $$

CREATE PROCEDURE `random_rows`(IN tab_name VARCHAR(64), IN num_rows INT)
BEGIN

SET @t = CONCAT('SET @max=(SELECT MAX(id) FROM ',tab_name,')');
PREPARE stmt FROM @t;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

SET @t = CONCAT(
    'SELECT * FROM ',
    tab_name,
    ' WHERE id>FLOOR(RAND()*@max) LIMIT ',
    num_rows);

PREPARE stmt FROM @t;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END
$$

then

CALL [schema name].random_rows([table name], n);

使用这个查询:

select floor(RAND() * (SELECT MAX(key) FROM table)) from table limit 10

查询时间:0.016秒

如果键之间没有间隙而且都是数字你可以计算随机数然后选择这些行。但事实可能并非如此。

所以一种解决方案是:

SELECT * FROM table WHERE key >= FLOOR(RAND()*MAX(id)) LIMIT 1

这将确保你在键的范围内得到一个随机数然后你选择下一个更大的最佳值。 你必须这样做10次。

然而，这并不是随机的，因为你的钥匙很可能不是均匀分布的。

这真的是一个大问题，不容易解决满足所有的要求，MySQL的rand()是最好的，如果你真的想要10个随机行。

然而，还有另一种解决方案，它速度很快，但涉及到随机性时也需要权衡，但可能更适合你。在这里阅读:我如何优化MySQL的ORDER BY RAND()函数?

问题是你需要它有多随机。

你能多解释一下吗?这样我才能给你一个好的解决办法。

例如，我合作的一家公司有一个解决方案，他们需要非常快的绝对随机性。最后，他们用随机值预填充数据库，这些随机值是从降序选择的，然后再次设置为不同的随机值。

如果你几乎没有更新，你也可以填充一个递增的id，这样你就没有间隙，只是可以在选择之前计算随机键…这取决于用例!

我改进了@Riedsio的答案。这是我在一个有间隙的大型均匀分布表上能找到的最有效的查询(测试从一个有> 2.6B行的表中获得1000个随机行)。

(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max := (SELECT MAX(id) FROM table)) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
(SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1)

让我来解释一下发生了什么。

(SELECT MAX(id) FROM table) 我在计算并保存最大值。对于非常大的表，每次需要一行时计算MAX(id)都会有轻微的开销 SELECT FLOOR(rand() * @max) + 1 as rand) 获取一个随机id SELECT id FROM table INNER JOIN(… 这就填补了空白。基本上，如果你在间隙中随机选择一个数字，它就会选择下一个id。假设间隙是均匀分布的，这应该不是问题。

进行联合可以帮助您将所有内容放入一个查询中，从而避免进行多个查询。它还可以节省计算MAX(id)的开销。根据您的应用程序，这可能非常重要，也可能无关紧要。

注意，这只获取id，并以随机顺序获取它们。如果你想做更高级的事情，我建议你这样做:

SELECT t.id, t.name -- etc, etc
FROM table t
INNER JOIN (
    (SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max := (SELECT MAX(id) FROM table)) + 1 as rand) r on id > rand LIMIT 1) UNION
    (SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
    (SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
    (SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
    (SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
    (SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
    (SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
    (SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
    (SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1) UNION
    (SELECT id FROM table INNER JOIN (SELECT FLOOR(RAND() * @max) + 1 as rand) r on id > rand LIMIT 1)
) x ON x.id = t.id
ORDER BY t.id

我认为这是一个简单但更快的方法，我在现场服务器上测试了它，与上面的几个答案相比，它更快。

 SELECT * FROM `table_name` WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table_name` ) ORDER BY id LIMIT 30; 

//对一个130行的表花费0.0014秒

SELECT * FROM `table_name` WHERE 1 ORDER BY RAND() LIMIT 30

//对130行的表花费0.0042秒

 SELECT name
FROM random AS r1 JOIN
   (SELECT CEIL(RAND() *
                 (SELECT MAX(id)
                    FROM random)) AS id)
    AS r2
WHERE r1.id >= r2.id
ORDER BY r1.id ASC
LIMIT 30

//对130行的表花费0.0040秒

我得到了快速查询(大约0.5秒)，cpu很慢，在一个400K寄存器的MySQL数据库中随机选择10行，非缓存2Gb大小。在MySQL中快速选择随机行

$time= microtime_float();

$sql='SELECT COUNT(*) FROM pages';
$rquery= BD_Ejecutar($sql);
list($num_records)=mysql_fetch_row($rquery);
mysql_free_result($rquery);

$sql="SELECT id FROM pages WHERE RAND()*$num_records<20
   ORDER BY RAND() LIMIT 0,10";
$rquery= BD_Ejecutar($sql);
while(list($id)=mysql_fetch_row($rquery)){
    if($id_in) $id_in.=",$id";
    else $id_in="$id";
}
mysql_free_result($rquery);

$sql="SELECT id,url FROM pages WHERE id IN($id_in)";
$rquery= BD_Ejecutar($sql);
while(list($id,$url)=mysql_fetch_row($rquery)){
    logger("$id, $url",1);
}
mysql_free_result($rquery);

$time= microtime_float()-$time;

logger("num_records=$num_records",1);
logger("$id_in",1);
logger("Time elapsed: <b>$time segundos</b>",1);