As the title says,i need to mass-assign products to a category and from the admin i can only edit one product at a time; i dont know why it just doesnt work to mass add them
I would shy away from tackling this problem from the database side of things. If you do go that direction make sure and take lots of backups and do it during low usage.
The following thread on the Magento forum identifies the very same problem. One poster recommends a raw sql approach with example. Again, I would be careful - make sure you take backups.
The answer I like best from the thread (posted by Magento MVP):
Go into the category you don’t want them in, find the product list. Click the check boxes on the products you want to remove and select delete from the little dropdown.
Now go into the category where you do want them, go to the product list. Select the NO dropdown so it shows items not in the category. You might have to do a selective search to limit stuff and do it in a couple iterations. Click the check boxes and tell it to add stuff.