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  1. Another feature not present in the SQL standard is the trigger.

    Several existing systems have their own non-standard trigger features.

  2. A trigger is a statement that is automatically executed by the system as a side effect of a modification to the database.
  3. We need to
  4. For example, suppose that an overdraft is intended to result in the account balance being set to zero, and a loan being created for the overdraft amount.

    The trigger actions for tuple t with a negative balance are then

    Note that this is not a good example. What would happen if the customer already had a loan?
  5. SQL-92 does not include triggers. To write this trigger in terms of the original System R trigger:

     define trigger  overdraft 

    on update of account T

    (if new T.balance < 0

    then (insert into loan values

    (T.bname, T.account#, - new T.balance)

    insert into borrower

    (select cname, account#

    from depositor

    where T.coount# = depositor.account#)

    update account S

    set S.balance = 0

    where S.account# = T.account# ))

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