Triggers

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
   • 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 with a negative balance are then

   • Insert a new tuple in the borrow relation with

     

   • We need to negate balance to get amount, as balance is negative.
   • Set to 0.
   Note that this is not a good example. What would happen if the customer already had a loan?

5. To write this trigger in terms of the original System R trigger: