Can anyone tell me whats the advantage of using weak entities, is it a principle to get the data structure more close to the reality? there are performance gains at some level when using it?

I understand the concept but I was wondering whats the advantage of using it besides more id space related to the primary key depending on other table's primary key?

the advantage is that a weak entity uses a foreign key to another entity as part of its own primary key

it has nothing to do with space and everything to do with accurately modelling the data

hmm I see, so it is a questio of modeling after all, ok thanks for your answer, apreciated ^^

by the way, if I want to use one weak entity say companies & products

and the product goes something like

create table product(
`id` int(11) UNSIGNED NOT NULL,
`company_id int (11) UNSIGNED NOT NULL,
....
PRIMARY KEY( `id`, `company_id`)
...
whats the correct way of doing this, with auto_increment on the id ?

I was wondering if it is possible to have something like:
company 1 product 1 /
company 2 product 1
company 1 product 2
...
etc
(only incrementing within the same company automatically)
is it possible or is not a good practice?