Basic Concepts

1. Dangling tuples.
   • Consider a pair of relations r(R) and s(S), and the natural join .
   • There may be a tuple in r that does not join with any tuple in s.
   • That is, there is no tuple in s such that .
   • We call this a dangling tuple.
   • Dangling tuples may or may not be acceptable.
2. Suppose there is a tuple in the account relation with the value ``Lunartown'', but no matching tuple in the branch relation for the Lunartown branch.

   This is undesirable, as should refer to a branch that exists.

   Now suppose there is a tuple in the branch relation with ``Mokan'', but no matching tuple in the account relation for the Mokan branch.

   This means that a branch exists for which no accounts exist. This is possible, for example, when a branch is being opened. We want to allow this situation.

3. Note the distinction between these two situations: bname is the primary key of branch, while it is not for account.

   In account, bname is a foreign key, being the primary key of another relation.

   • Let and be two relations with primary keys and respectively.
   • We say that a subset of is a foreign key referencing in relation if it is required that for every tuple in there must be a tuple in such that

   • We call these requirements referential integrity constraints.
   • Also known as subset dependencies, as we require