Syntax:
RAND([N])

Returns a random floating-point value v in the range 0 <= v < 1.0. To
obtain a random integer R in the range i <= R < j, use the expression
FLOOR(i + RAND() * (j − i)). For example, to obtain a random integer
in the range the range 7 <= R < 12, use the following statement:

SELECT FLOOR(7 + (RAND() * 5));

If an integer argument N is specified, it is used as the seed value:

o With a constant initializer argument, the seed is initialized once
  when the statement is prepared, prior to execution.

o With a nonconstant initializer argument (such as a column name), the
  seed is initialized with the value for each invocation of RAND().

One implication of this behavior is that for equal argument values,
RAND(N) returns the same value each time, and thus produces a
repeatable sequence of column values. In the following example, the
sequence of values produced by RAND(3) is the same both places it
occurs.

URL: http://dev.mysql.com/doc/refman/5.7/en/mathematical-functions.html

