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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)
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原文地址：http://www.cnblogs.com/zejin2008/p/4710364.html
