Data sorting of csv files

I have a database of 1.x million business's. These business's are all sorted by there category. I have another file that contains all possible categories. Categories 1, 2, 3. The database only has category 3 of all businesses. I need someone to add category 1 and 2 to all the businesses in the database. That means you will have to cross reference the two files and when there is a match, copy the matched category 1 & 2 into the database file that only contains category 3 for all businesses.