title: Minimum number of candidate keys for a relation?
body: <p>My question is, is it necessary for a relation/table in database to have a candidate key and hence a primary key? Is it possible to have a relation where a row cannot be uniquely identified by any combination of attributes?</p>
  
  <p>If no, why? And if yes, then how does a DBMS make operations like search, delete etc, efficient?</p>