Syntax:
MIN([DISTINCT] expr), MAX([DISTINCT] expr)

Returns the minimum or maximum value of expr. MIN() and MAX() may take
a string argument; in such cases they return the minimum or maximum
string value. See
http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html. The DISTINCT
keyword can be used to find the minimum or maximum of the distinct
values of expr, however, this produces the same result as omitting
DISTINCT.

MIN() and MAX() return NULL if there were no matching rows.

URL: http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html

