Syntax:
LOG(X), LOG(B,X)

If called with one parameter, this function returns the natural
logarithm of X. If X is less than or equal to 0, then NULL is returned.

The inverse of this function (when called with a single argument) is
the EXP() function.

URL: http://dev.mysql.com/doc/refman/5.1/en/mathematical-functions.html

