Can the Primary Key also be Primary key in another table

Question

Hi all,
I am working on a project. The project specification says:
Project table contains details of projects and the project number *(proj_no)* is the key.
project ( proj_no , proj_date, proj_desc, proj_type, proj_status, s_no, cust_no)
purchase_order table contains details of the purchase orders for each project and the combination of project
number and a purchase order number *(proj_no, po_no) form the key.*
purchase_order ( proj_no , po_no, po_date)
How can I have proj_no in the project table and in the purchase_order as primary keys?
If I create a composite key in the purchase_order table making the ( proj_no, po_no) as primary key. How can I make the project_no in the project table a primary key too?
Is there a solution for it?
CREATE TABLE purchase_order(
po_no NUMBER(5) NOT NULL,
proj_no NUMBER(5) NOT NULL,
po_date date,
CONSTRAINT p_o_po_no_proj_no_pk PRIMARY KEY (po_no,proj_no));
Thanks!!!

Answer 1

Hi,
I think I'd set the project no in the projects table a primary key, and project no in the purchase orders table a foreign key constraint to the projects table, combined with a unique, not null constraint in the table.
That, in addition to a unique, not null constraint on the purchase orders column, and a composite primary key should fulfill the requirements.
Best of luck!
Johan