RAND（）随机生成 0-1 的随机数

mysql> select RAND(),RAND(3),RAND(0),RAND(-2);
+---------------------+--------------------+---------------------+--------------------+
| RAND() | RAND(3) | RAND(0) | RAND(-2) |
+---------------------+--------------------+---------------------+--------------------+
| 0.10204030210342287 | 0.9057697559760601 | 0.15522042769493574 | 0.6548542125661431 |
+---------------------+--------------------+---------------------+--------------------+
1 row in set (0.03 sec)