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

给你提供一个思路你看是否可行；

1.随机数不用MYSQL生成，用程序（如PHP）生成，范围在ID的最大和最小之间；

2.根据生成的随机数去数据库直接查询，如果没有就重复一次 *** 作；

rand() 随机生成 0 - 1的浮点数 , 常与其他函数结合使用 ,比如 ceiling,floor,LPAD 等

如果要指定指定范围的随机整数的话，需要用这个公式FLOOR(i + RAND() * j),比如 

# 生成 7 - 11的随机数  SELECT FLOOR(7 + (RAND() * 5))

floor 地板取小于该值的最大整数 ,比如 0

mysql>select floor(1.23),floor(-1.23)

   1           -2 

ceiling 则相反,向上取整,取大于该值的最小整数 ,比如

SELECT CEILING(1.23)# 2

SELECT CEIL(-1.23)# -1

lpad 是左填充, 用法如下 :

LPAD(RAND()*31 + 1,2,'0')) # 取01-31的随机整数 ,保留两位,如果是一位,左边填0

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