要从tablename表中随机提取一条记录，大家一般的写法就是：

SELECT * FROM tablename ORDER BY RAND() LIMIT 1。

mysql> SELECT RAND();
+-------------------+
| RAND()            |
+-------------------+
| 0.048264377795406 |
+-------------------+
1 row in set (0.00 sec)
 
mysql> SELECT RAND();
+-------------------+
| RAND()            |
+-------------------+
| 0.557701361885016 |
+-------------------+
1 row in set (0.00 sec)
 
mysql> SELECT RAND();
+-------------------+
| RAND()            |
+-------------------+
| 0.643713706772508 |
+-------------------+
1 row in set (0.00 sec)
 
mysql> SELECT RAND();
+-------------------+
| RAND()            |
+-------------------+
| 0.545464478941136 |
+-------------------+
1 row in set (0.00 sec)

RAND() 函数返回的是一个小于1的随机数　　

但是，后来查了一下MYSQL的官方手册，里面针对RAND()的提示大概意思就是，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。但是在MYSQL 3.23版本中，仍然可以通过ORDER BY RAND()来实现随机。

但是真正测试一下才发现这样效率非常低。一个15万余条的库，查询5条数据，居然要8秒以上。查看官方手册，也说rand()放在ORDER BY 子句中会被执行多次，自然效率及很低。
You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times.

搜索Google，网上基本上都是查询max(id) * rand()来随机获取数据。

SELECT *
FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * (SELECT MAX(id) FROM `table`)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id ASC LIMIT 5;

但是这样会产生连续的5条记录。解决办法只能是每次查询一条，查询5次。即便如此也值得，因为15万条的表，查询只需要0.01秒不到。

下面的语句采用的是JOIN，mysql的论坛上有人使用

SELECT *
FROM `table`
WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table` )
ORDER BY id LIMIT 1;

测试了一下，需要0.5秒，速度也不错，但是跟上面的语句还是有很大差距。

于是把语句改写了一下。

SELECT * FROM `table`
WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `table`)))
ORDER BY id LIMIT 1;

这下，效率又提高了，查询时间只有0.01秒

最后，再把语句完善一下，加上MIN(id)的判断。在最开始测试的时候，就是因为没有加上MIN(id)的判断，结果有一半的时间总是查询到表中的前面几行。
完整查询语句是：

SELECT * FROM `table`
WHERE id >= (SELECT floor( RAND() * ((SELECT MAX(id) FROM `table`)
-(SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`)))
ORDER BY id LIMIT 1;

SELECT *
FROM `table` AS t1 JOIN (SELECT ROUND(RAND() * ((SELECT MAX(id) FROM `table`)
-(SELECT MIN(id) FROM `table`))+(SELECT MIN(id) FROM `table`)) AS id) AS t2
WHERE t1.id >= t2.id
ORDER BY t1.id LIMIT 1;

最后对这两个语句进行分别查询10次，
前者花费时间 0.147433 秒
后者花费时间 0.015130 秒
看来采用JOIN的语法比直接在WHERE中使用函数效率还要高很多。

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Mysql 随机查询10条数据效率最快的查询方法

1）使用join 和 rand() 耗时 0.009

SELECT
    *
FROM
    `t_topic` AS t1
JOIN (
    SELECT
        ROUND(
            RAND() * (
                (SELECT MAX(id) FROM `t_topic`) - (SELECT MIN(id) FROM `t_topic`)
            ) + (SELECT MIN(id) FROM `t_topic`)
        ) AS id
) AS t2
WHERE
    t1.id >= t2.id
ORDER BY
    t1.id
LIMIT 10;

2) 下面这条比上面那个还要慢几秒

SELECT * FROM `t_topic` 
WHERE id >= (SELECT floor( RAND() * ((SELECT MAX(id) FROM `t_topic`)-
(SELECT MIN(id) FROM `t_topic`)) + (SELECT MIN(id) FROM `t_topic`)))  
ORDER BY id LIMIT 10;

3）或者使用下面这个也可以测试也是耗时0.0012

SELECT
    *
FROM
    `t_topic`
WHERE
    id >= (
        SELECT
            floor(
                RAND() * (SELECT MAX(id) FROM `t_topic`)
            )
    )
ORDER BY
    id
LIMIT 10;

--end--

mysql随机查询一条或多条数据

Mysql 随机查询10条数据效率最快的查询方法