Syntax:
MD5(str)

Calculates an MD5 128-bit checksum for the string. The value is
returned as a string of 32 hex digits, or NULL if the argument was
NULL. The return value can, for example, be used as a hash key. See the
notes at the beginning of this section about storing hash values
efficiently.

As of MySQL 5.5.3, the return value is a nonbinary string in the
connection character set. Before 5.5.3, the return value is a binary
string; see the notes at the beginning of this section about using the
value as a nonbinary string.

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

