title: why do we need weak entity instead of making it strong entity?
body: <p>For example, I have 2 entities: book and copy with 1-n relationship as a book can have many its copies.</p>
  
  <p>if copy is a strong entity,</p>
  
  <p>book(PK_ISBN#, title, edition, date) 
  copy(PK_copy#, condition, FK_ISBN#)</p>
  
  <p>if copy is a weak entity,</p>
  
  <p>book(PK_ISBN#, title, edition, date) 
  copy(ISBN#, copy#, condition) 
  Primary key (ISBN#, copy#)
  Foreign key ISBN# references book(PK_ISBN#)</p>
  
  <p><strong>Question:</strong> why would the copy entity be the weak entity instead of strong entity when both cases, I think, are similar.</p>
  
  <p>P/S: one more question: How can we model partial or total participation constraint in SQL code.</p>