MySQL产生随机数

MYSQL 取随机数

2010年04月26日 星期一 09:48

mysql 取随机数

--对一个表取任意随机数

SELECT *

FROM TMP_XF_TEST

WHERE ID >= (SELECT FLOOR(RAND() * (SELECT MAX(ID) FROM TMP_XF_TEST)))

order by id LIMIT 1

--有条件性的取随机数

SELECT *

FROM TMP_XF_TEST

WHERE ID >= (SELECT FLOOR(RAND() *

((SELECT MAX(ID) FROM TMP_XF_TEST WHERE GID = 9) -

(SELECT MIN(ID) FROM TMP_XF_TEST WHERE GID = 9))) +

(SELECT MIN(ID) FROM TMP_XF_TEST WHERE GID = 9))

AND GID = 9

ORDER BY ID LIMIT 1

--gid上存在索引

或者

SELECT *

FROM TMP_XF_TEST AS t1 JOIN

(SELECT ROUND(RAND() * ((SELECT MAX(id) FROM TMP_XF_TEST WHERE GID = 9)-(SELECT MIN(id) FROM TMP_XF_TEST WHERE GID = 9))

+(SELECT MIN(id) FROM TMP_XF_TEST WHERE GID = 9)) AS id) AS t2

WHERE t1.id >= t2.id AND t1.GID = 9

ORDER BY t1.id LIMIT 1

#########

不要用下面的杯具写法

mysql>insert into tmp_xf_test(user_nick,gid,item_id,gmt_create,gmt_modified,memo)

->select user_nick,gid,item_id,gmt_create,gmt_modified,memo from tmp_xf_test

Query OK, 165888 rows affected (9.65 sec)

Records: 165888 Duplicates: 0 Warnings: 0

mysql>SELECT *

->FROM `tmp_xf_test`

->WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `tmp_xf_test` )

->ORDER BY id LIMIT 1

+-----+-----------+-----+---------+---------------------+---------------------+--------------------+

| id | user_nick | gid | item_id | gmt_create | gmt_modified| memo |

+-----+-----------+-----+---------+---------------------+---------------------+--------------------+

| 467 | 玄风 | 9 | 123 | 2010-04-26 14:56:39 | 2010-04-26 14:56:39 | 玄风测试使用的数据 |

+-----+-----------+-----+---------+---------------------+---------------------+--------------------+

1 row in set (51.12 sec)

mysql>explain SELECT *

->FROM `tmp_xf_test`

->WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `tmp_xf_test` )

->ORDER BY id LIMIT 1\G

*************************** 1. row ***************************

id: 1

select_type: PRIMARY

table: tmp_xf_test

type: index

possible_keys: NULL

key: PRIMARY

key_len: 8

ref: NULL

rows: 1

Extra: Using where

*************************** 2. row ***************************

id: 2

select_type: UNCACHEABLE SUBQUERY

table: tmp_xf_test

type: index

possible_keys: NULL

key: idx_tmp_xf_test_gid

key_len: 4

ref: NULL

rows: 331954

Extra: Using index

2 rows in set (0.01 sec)

---

mysql>SELECT * FROM `tmp_xf_test` t1 join

->(SELECT FLOOR( MAX(id) * RAND()) as id FROM `tmp_xf_test` ) as t2

->where t1.id >=t2.id

->ORDER BY t1.id LIMIT 1

+-------+-----------+-----+---------+---------------------+---------------------+--------------------+-------+

| id| user_nick | gid | item_id | gmt_create | gmt_modified| memo | id|

+-------+-----------+-----+---------+---------------------+---------------------+--------------------+-------+

| 40311 | 玄风 | 9 | 123 | 2010-04-28 15:47:19 | 2010-04-28 15:47:19 | 玄风测试使用的数据 | 40311 |

+-------+-----------+-----+---------+---------------------+---------------------+--------------------+-------+

1 row in set (0.14 sec)

##############

mysql>SELECT * FROM `tmp_xf_test`

->WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `tmp_xf_test`)))

->ORDER BY id LIMIT 1

+------+-----------+-----+---------+---------------------+---------------------+--------------------+

| id | user_nick | gid | item_id | gmt_create | gmt_modified| memo |

+------+-----------+-----+---------+---------------------+---------------------+--------------------+

| 1352 | 玄风 | 9 | 123 | 2010-04-28 15:47:19 | 2010-04-28 15:47:19 | 玄风测试使用的数据 |

+------+-----------+-----+---------+---------------------+---------------------+--------------------+

1 row in set (0.00 sec)

mysql>explain SELECT * FROM `tmp_xf_test`

->WHERE id >= (SELECT floor(RAND() * (SELECT MAX(id) FROM `tmp_xf_test`)))

->ORDER BY id LIMIT 1\G

*************************** 1. row ***************************

id: 1

select_type: PRIMARY

table: tmp_xf_test

type: index

possible_keys: NULL

key: PRIMARY

key_len: 8

ref: NULL

rows: 1

Extra: Using where

*************************** 2. row ***************************

id: 3

select_type: SUBQUERY

table: NULL

type: NULL

possible_keys: NULL

key: NULL

key_len: NULL

ref: NULL

rows: NULL

Extra: Select tables optimized away

2 rows in set, 1 warning (0.00 sec)

对应的另外一种杯具写法是：

SELECT *

FROM TMP_XF_TEST

WHERE ID >= (SELECT FLOOR(RAND() * (MAX(ID) - MIN(ID))) + MIN(ID) MID

FROM TMP_XF_TEST

WHERE GID = 9)

AND GID = 9 LIMIT 1

mysql的rand函数可以生成一个0到1之间的随机数，进行一定的放大即可得到一个随机数。再通过条件查询来限制新随机数没有在表中出现过。如下所示：

SELECT FLOOR(RAND() * 99999) AS random_numFROM numbers

WHERE "random_num" NOT IN (SELECT my_number FROM numbers)

LIMIT 1

在mysql中随机生成一些范围内的整数，可以借助mysql提供的二个函数来实现。

它们是floor()与rand()函数，下面分别介绍下。

1、floor(f)

返回一个不大于f的最大整数

2、rand(),rand(n)

返回一个随机浮点值 v ，范围在 0 到1 之间 (即, 其范围为 0 ≤ v ≤ 1.0)。若已指定一个整数参数 N ，则它被用作种子值，用来产生重复序列。

若要在i ≤ R ≤ j 这个范围得到一个随机整数R ，需要用到表达式 FLOOR(i + RAND() * (j – i + 1))。

例如， 在7到 12 的范围（包括7和12）内得到一个随机整数，使用语句：

SELECT FLOOR(7 + (RAND() * 6))

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