Syntax:
LN(X)

Returns the natural logarithm of X; that is, the base-e logarithm of X.
If X is less than or equal to 0.0E0, the function returns NULL and (as
of MySQL 5.7.4) a warning "Invalid argument for logarithm" is reported.

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

