title: Primary key for a relation
body: <p>I have to relate 2 entities, for a database representing a zoo. 
  It is shift patterns for zoo keepers looking after certain enclosures. </p>
  
  <p>We have 
  Keeper_Number (FK from "Keeper" table)
  Enclosure_Number (FK from "Enclosure" table) 
  Shift start
  Shift end</p>
  
  <p>Obviously we don't want one person to be able to work in two places at the same time. My question is, am I best making all of these primary keys, so the combination is always unique, or just applying a unique constraint and leaving the table with no foreign key? Apologies for no DDL, my understanding is very limited and I have primarily used the UI of Miscrosoft sequel server express. </p>
  
  <p>Thanks!</p>