MySQL
RAND()函数调用可以在0和1之间产生一个随机数：
?
1
2
3
4
5
6
7
mysql
SELECT
RAND(
),
RAND(
),
RAND(
)
+------------------+-----------------+------------------+
|
RAND(
)
|
RAND(
)
|
RAND(
)
|
+------------------+-----------------+------------------+
|
0.45464584925645
|
0.1824410643265
|
0.54826780459682
|
+------------------+-----------------+------------------+
1
row
in
set
(0.00
sec)
当调用一个整数参数时，RAND()使用该随机数发生器种子值。每次在给定值种子生成，RAND()会产生一个可重复的一系列数字：
?
1
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7
mysql
SELECT
RAND(1),
RAND(
),
RAND(
)
+------------------+------------------+------------------+
|
RAND(1
)
|
RAND(
)
|
RAND(
)
|
+------------------+------------------+------------------+
|
0.18109050223705
|
0.75023211143001
|
0.20788908117254
|
+------------------+------------------+------------------+
1
row
in
set
(0.00
sec)
可以使用ORDER
BY
RAND()，随机组行或值如下：
要了解ORDER
BY
RAND()函数,
假设EMPLOYEE_TBL的表有以下记录：
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7
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13
mysql
SELECT
*
FROM
employee_tbl
+------+------+------------+--------------------+
|
id
|
name
|
work_date
|
daily_typing_pages
|
+------+------+------------+--------------------+
|
1
|
John
|
2007-01-24
|
250
|
|
2
|
Ram
|
2007-05-27
|
220
|
|
3
|
Jack
|
2007-05-06
|
170
|
|
3
|
Jack
|
2007-04-06
|
100
|
|
4
|
Jill
|
2007-04-06
|
220
|
|
5
|
Zara
|
2007-06-06
|
300
|
|
5
|
Zara
|
2007-02-06
|
350
|
+------+------+------------+--------------------+
7
rows
in
set
(0.00
sec)
现在使用以下目录：
?
1
2
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5
6
7
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12
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15
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18
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mysql
SELECT
*
FROM
employee_tbl
ORDER
BY
RAND()
+------+------+------------+--------------------+
|
id
|
name
|
work_date
|
daily_typing_pages
|
+------+------+------------+--------------------+
|
5
|
Zara
|
2007-06-06
|
300
|
|
3
|
Jack
|
2007-04-06
|
100
|
|
3
|
Jack
|
2007-05-06
|
170
|
|
2
|
Ram
|
2007-05-27
|
220
|
|
4
|
Jill
|
2007-04-06
|
220
|
|
5
|
Zara
|
2007-02-06
|
350
|
|
1
|
John
|
2007-01-24
|
250
|
+------+------+------------+--------------------+
7
rows
in
set
(0.01
sec)
mysql
SELECT
*
FROM
employee_tbl
ORDER
BY
RAND()
+------+------+------------+--------------------+
|
id
|
name
|
work_date
|
daily_typing_pages
|
+------+------+------------+--------------------+
|
5
|
Zara
|
2007-02-06
|
350
|
|
2
|
Ram
|
2007-05-27
|
220
|
|
3
|
Jack
|
2007-04-06
|
100
|
|
1
|
John
|
2007-01-24
|
250
|
|
4
|
Jill
|
2007-04-06
|
220
|
|
3
|
Jack
|
2007-05-06
|
170
|
|
5
|
Zara
|
2007-06-06
|
300
|
+------+------+------------+--------------------+
7
rows
in
set
(0.00
sec)
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
RAND()RAND(N)
返回在范围0到1.0内的随机浮点值。
如果一个整数参数N被指定，它被用作种子值。
每个种子产生的随机数序列是不同的
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