mysql rand()产生随机整数范围及方法

根据官方文档，rand()的取值范围为[0,1)

在Mysql中可以执行如下命令查看：

mysql> ? rand
Name: ‘RAND‘
Description:
Syntax:
RAND(), RAND(N)

Returns a random floating-point value v in the range 0 <= v < 1.0. If a
constant integer argument N is specified, it is used as the seed value,
which produces a repeatable sequence of column values. In the following
example, note that the sequences of values produced by RAND(3) is the
same both places where it occurs.

URL: http://dev.mysql.com/doc/refman/5.6/en/mathematical-functions.html

Examples:
mysql> CREATE TABLE t (i INT);
Query OK, 0 rows affected (0.42 sec)

mysql> INSERT INTO t VALUES(1),(2),(3);
Query OK, 3 rows affected (0.00 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> SELECT i, RAND() FROM t;
+------+------------------+
| i    | RAND()           |
+------+------------------+
|    1 | 0.61914388706828 |
|    2 | 0.93845168309142 |
|    3 | 0.83482678498591 |
+------+------------------+
3 rows in set (0.00 sec)

mysql> SELECT i, RAND(3) FROM t;
+------+------------------+
| i    | RAND(3)          |
+------+------------------+
|    1 | 0.90576975597606 |
|    2 | 0.37307905813035 |
|    3 | 0.14808605345719 |
+------+------------------+
3 rows in set (0.00 sec)

mysql> SELECT i, RAND() FROM t;
+------+------------------+
| i    | RAND()           |
+------+------------------+
|    1 | 0.35877890638893 |
|    2 | 0.28941420772058 |
|    3 | 0.37073435016976 |
+------+------------------+
3 rows in set (0.00 sec)

mysql> SELECT i, RAND(3) FROM t;
+------+------------------+
| i    | RAND(3)          |
+------+------------------+
|    1 | 0.90576975597606 |
|    2 | 0.37307905813035 |
|    3 | 0.14808605345719 |
+------+------------------+
3 rows in set (0.01 sec)

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， 若要在7 到 12 的范围（包括7和12）内得到一个随机整数, 可使用以下语句：
SELECT FLOOR(7 + (RAND() * 6));

另外，也可以用round四舍五入函数来实现，考虑到最前与最后的取值概率会与中间的不相等，故加上0.5来消除这种概率上的差异，达到均匀分布：

产生[i,j]范围正整数：select round(rand()*(j-i+1)+i-0.5)

若要在7 到 12 的范围（包括7和12）内得到一个随机整数, 可使用以下语句：
SELECT round( (RAND() * 6+6.5));

如果产生负整数的话得注意最左边的值有可能会超过你的要求：

mysql> SELECT round(-0.5);
+-------------+
| round(-0.5) |
+-------------+
|          -1 |
+-------------+
1 row in set (0.00 sec)

mysql> SELECT round(-0.4);
+-------------+
| round(-0.4) |
+-------------+
|           0 |
+-------------+
1 row in set (0.00 sec)