Usage of Primary and Foreign keys in an EER diagram

In my db I have three tables (I have more but for case is equal, users can be companies or single people).

• Users has a primary key id_user;
• Company has a primary key id_company and a foreign key users_id_user;
• job_offers has a primary key id_job_offers and two foreign keys: company_id_company and company_users_id_user.

My questions are:

1. Does a primary key make sense in job_offers? I don't think that there is a reason for it.
2. job_offers has two foreign keys, one related to company and other to users. Is there a problem with this? Does there exist another way to accomplish the same task?

Does a primary key make sense in job_offers? I don't think that there is a reason for it.

Yes . I agree that every table should have their own PK. Should each and every table have a primary key?

I have more but for case is equal, users can be companies or single people

job_offers has two foreign keys, one related to company and other to users. Is there a problem with this? Does there exist another way to accomplish the same task?

The system have two types of users: normal user (person) and company user. The job_offers is a table that save job offers from a company. If a company user want to post a job , a record will be inserted to the job_offers table . Then once the normal user get this job offer , the job_offers.company_user_id_user will be assigned to this normal user 's userid.

But from your ER diagram , Company.users_id_user is the PK , which cannot be null , and this PK is used in the job_offers.company_users_id_user as a FK. So job_offers.company_users_id_user also cannot be null .

As a result , it cannot handle the situation that a company user just post a job and before a normal user gets this job offer or no one gets this job offer eventually .In this case, job_offers.company_users_id_user should set to null , which violates the job_offers.company_users_id_user 's not null constraint.

I will accomplish the same task using this design:

Users
=================
id_user (PK)
email 
activation
password

Company
=================
id_company (PK)
activities 
foundation 
user_id (FK to Users)
description

job_offer
=================
id_job_offer (PK)
id_company (FK to Company)
description_offer 
tags

user_offer
=================
id (PK)
user_id (FK to Users)
job_offer_id (FK to job_offer)