Basic Concepts



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Basic Concepts

  1. Dangling tuples.
  2. Suppose there is a tuple in the deposit 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 deposit 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 deposit.

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



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Last Update: Mon Oct 16 17:01:17 PDT 1995