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 e ect of a modi cation to the database.

3. We need to

• Specify the conditions under which the trigger is executed.
• Specify the actions to be taken by the trigger.

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

• Insert a new tuple s in the borrow relation with
  s[bname] = t[bname]
  s[loan#] = t[account#]
  s[amount] = ô€€€t[balance]
  s[cname] = t[cname]
• We need to negate balance to get amount, as balance is negative.
• Set t[balance] to 0.

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# ))