Syntax:
DECODE(crypt_str,pass_str)

DECODE() decrypts the encrypted string crypt_str using pass_str as the
password. crypt_str should be a string returned from ENCODE().

*Note*: The ENCODE() and DECODE() functions are deprecated in MySQL
5.7, will be removed in a future MySQL release, and should no longer be
used. Consider using AES_ENCRYPT() and AES_DECRYPT() instead.

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

