Syntax:
MAX([DISTINCT] expr)

Returns the maximum value of expr. MAX() may take a string argument; in
such cases, it returns the maximum string value. See
http://dev.mysql.com/doc/refman/5.5/en/mysql-indexes.html. The DISTINCT
keyword can be used to find the maximum of the distinct values of expr,
however, this produces the same result as omitting DISTINCT.

If there are no matching rows, MAX() returns NULL.

URL: http://dev.mysql.com/doc/refman/5.5/en/group-by-functions.html

