Question

Take these tables for example.

Item
    id
    description
    category

Category
    id
    description 

An item can belong to many categories and a category obviously can be attached to many items.

How would the database be created in this situation? I’m not sure. Someone said create a third table, but do I need to do that? Do I literally do a

create table bla bla

for the third table?

Answer 1

Yes, you need to create a third table with mappings of ids, something with columns like:

 item_id     (Foreign Key)
 category_id (Foreign Key)

edit: you can treat item_id and category_id as a primary key, they uniquely identify the record alone. In some applications I’ve found it useful to include an additional numeric identifier for the record itself, and you might optionally include one if you’re so inclined

Think of this table as a listing of all the mappings between Items and Categories. It’s concise, and it’s easy to query against.

edit: removed (unnecessary) primary key.