Some kind of checksum

I'm using MySQL and that is the context with which I write this email. I have a buch of text files that I want to add to a database so we can add all kinds of search functionality for those files. Each would be stored in either a TEXT or BLOB field. These files can have all kinds of special characters ( ", ', etc) so it would be necessary that I use addslashes(). What I would like to do is make it so that I can check to see if the file has already been added to the database. I'm having a problem where if the text is too large, I can't query to see if the contents of the current file equals the contents of the field I'm storing it in. Is there any way that I can do some kind of checksum on both the contents of the file and the contents of the field to determine if that file has already been added? I'm trying to make it so that I don't have duplicate files in the DB. Has anyone done something like this? Chris