Translucent databases

I understand the need to hash a username and/or a password. Indeed, such things as credit card data and SSN numbers. But if the database is to be queried to output results, can any of this be hashed and if so, how can it be decrypted, for output.

If anyone knows of a good up-to-date tutorial/article and can send me link I would appreciate it. I have read the O’Reilly article on it but it is from 2002.