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Basic Concepts

  1. Dangling tuples.
  2. Suppose there is a tuple tex2html_wrap_inline940 in the account relation with the value tex2html_wrap_inline942 ``Lunartown'', but no matching tuple in the branch relation for the Lunartown branch.

    This is undesirable, as tex2html_wrap_inline940 should refer to a branch that exists.

    Now suppose there is a tuple tex2html_wrap_inline946 in the branch relation with tex2html_wrap_inline948 ``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.

Osmar Zaiane
Tue Jun 9 15:12:55 PDT 1998