RAND(), RAND(N) returns a random floating-point value v in the range 0 <= v < 1.0. : RAND « Math Numeric Functions « MySQL Tutorial

1. MySQL Tutorial
2. Math Numeric Functions
3. RAND

If a constant integer argument N is specified, it is used as the seed value, which produces a repeatable sequence of column values.

mysql>
mysql> SELECT RAND();
+------------------+
| RAND()           |
+------------------+
| 0.72570215698863 |
+------------------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT RAND(20);
+------------------+
| RAND(20)         |
+------------------+
| 0.15888261251047 |
+------------------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT RAND(20);
+------------------+
| RAND(20)         |
+------------------+
| 0.15888261251047 |
+------------------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT RAND();
+------------------+
| RAND()           |
+------------------+
| 0.49934750655605 |
+------------------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT RAND();
+------------------+
| RAND()           |
+------------------+
| 0.31963109254803 |
+------------------+
1 row in set (0.00 sec)