In Perl or SQL, if I have a TABLE, and that table has, say, six things.

USERNAME
PASSWORD
EMAIL
DAYSACTIVE
POINTS
SIGNATURE

Now. Let's assume I only know what one of these are. USERNAME.

Can I through any of the standard libraries for these languages use a command like I can in c++ that effectively lets me tell the program to LIST for me, all the names that I don't know.

For instance (and this is not exercising proper syntax at all I know).

IF NOT IN_LIST "USERNAME"
THEN PRINT_ALL
ELSE RETURN 1

OR ANYTHING OF THE SORT?

I know that this is possible in some way, but I'd like to see exactly how it's done if anyone's familiar.