Syntax:
MIN([DISTINCT] expr)

Returns the minimum value of expr. MIN() may take a string argument; in
such cases, it returns the minimum string value. See
http://dev.mysql.com/doc/refman/5.7/en/mysql-indexes.html. The DISTINCT
keyword can be used to find the minimum of the distinct values of expr,
however, this produces the same result as omitting DISTINCT.

MIN() returns NULL if there were no matching rows.

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

