Syntax:

UNHEX(str)

For a string argument str, UNHEX(str) performs the inverse operation of
HEX(str). That is, it interprets each pair of characters in the
argument as a hexadecimal number and converts it to the character
represented by the number. The return value is a binary string.

URL: http://dev.mysql.com/doc/refman/5.1/en/string-functions.html

