There are various encryption methods available to MySQL and you will really need to find out how the data was encrypted before you can filter it back as unencrypted information. Be aware though that some encryption methods are one way (such as MD5 and SHA-1) and others are two way AES_ENCRYPT and ENCODE and their respective couterparts AES_DECRYPT and DECODE.
You will only get a satisfactory result if a two way encryption was used. These may also use a 'salt' which is an optional parameter to help achieve a more secure encryption.
More information can be found at the mysql site here
http://dev.mysql.com/doc/refman/5.0/en/encryption-functions.html
