Syntax:
HEX(str), HEX(N)

For a string argument str, HEX() returns a hexadecimal string
representation of str where each byte of each character in str is
converted to two hexadecimal digits. (Multi-byte characters therefore
become more than two 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/5.6/en/string-functions.html

