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A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity.

  • A candidate key of an entity set is a minimal super key
  • Customer-id is candidate key of customer
  • account-number is candidate key of account
  • Although several candidate keys may exist, one of the candidate keys is selected to be the primary key.

Keys for Relationship Sets

The combination of primary keys of the participating entity sets forms a super key of a relationship set.

(customer-id, account-number) is the super key of depositor



NOTE: this means a pair of entity sets can have at most one relationship in a particular relationship set.

E.g. if we wish to track all access-dates to each account by each customer, we cannot assume a relationship for each access. We can use a multivalued attribute though.


Must consider the mapping cardinality of the relationship set when deciding the what are the candidate keys.

Need to consider semantics of relationship set in selecting the primary key in case of more than one candidate key.

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