Select-option/Parameter & Primary Key

Dear Abaper,

    It this compulsory to have a Table fields define in Parameter/Select-option of Selection screen should be  Primary key of respective tables??

and

  If we used two different tables having common fields and if common field of Table1  is primary key and Table2  is not a primary key can we used that common field in Parameter/Select-option of selection screen?

Thanks & regards

Mahendra