SQL & PL/SQL

hi all,
i have confusion with the following.
let me say i have table called 'A' which is having columns 'company' and 'reference' which are primary key of the table 'A'. I have one more table called 'B' which is having same columns 'company' and 'reference' which are primary keys of 'A' and foreign keys of 'B'.
consider i am passing value to the column 'reference'.
please clear me what results may i get if i joined to tables and what will happen if i pass the value only to the table 'B'.

please clear me.

Thanks..