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/5.1/en/mathematical-functions.html

