I have an existent attribute and i want to add it to an attribute set. I drag and drop the attribute to a group. What database tables should i change to complete the process ?
You haven't required database table update for set attribute to attribute set. You have to do reindexing to get update in admin product page.
System -> Index Management
Reindexing all index and check.
I think the table you are looking for is
SELECT * FROM eav_attribute WHERE attribute_code = 'your attribute'
But the problem you are having, it could be that you are not loading the product Model properly, or you are unable to return the value of the attribute because it is not on the flat. First of all, enable the flat tables ‘Use flat catalog products’ in System -> Config -> Catalog And then, in your attribute, enable the ‘Used in product listing’ setting in Catalog > Attributes > Manage Attributes