Syntax:
HEX(str), HEX(N)

For a string argument str, HEX() returns a hexadecimal string
representation of str where each character in str is converted to two
hexadecimal digits. The inverse of this operation is performed by the
UNHEX() function.

For a numeric argument N, HEX() returns a hexadecimal string
representation of the value of N treated as a longlong (BIGINT) number.
This is equivalent to CONV(N,10,16). The inverse of this operation is
performed by CONV(HEX(N),16,10).

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

