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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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plement the construct by the equivalent not exists X such that not P (X) construct, which can be written in SQL We write create assertion sum-constraint check (not exists (select * from branch where (select sum(amount) from loan where loanbranch-name = branchbranch-name) >= (select sum(balance) from account where accountbranch-name = branchbranch-name))) create assertion balance-constraint check (not exists (select * from loan where not exists ( select * from borrower, depositor, account where loanloan-number = borrowerloan-number and borrowercustomer-name = depositorcustomer-name and depositoraccount-number = accountaccount-number and accountbalance >= 1000))) When an assertion is created, the system tests it for validity If the assertion is valid, then any future modi cation to the database is allowed only if it does not cause that assertion to be violated This testing may introduce a signi cant amount of overhead if complex assertions have been made Hence, assertions should be used with great care The high overhead of testing and maintaining assertions has led some system developers to omit support for general assertions, or to provide specialized forms of assertions that are easier to test
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A trigger is a statement that the system executes automatically as a side effect of a modi cation to the database To design a trigger mechanism, we must meet two requirements: 1 Specify when a trigger is to be executed This is broken up into an event that causes the trigger to be checked and a condition that must be satis ed for trigger execution to proceed 2 Specify the actions to be taken when the trigger executes The above model of triggers is referred to as the event-condition-action model for triggers The database stores triggers just as if they were regular data, so that they are persistent and are accessible to all database operations Once we enter a trigger into the database, the database system takes on the responsibility of executing it whenever the speci ed event occurs and the corresponding condition is satis ed
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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The McGraw Hill Companies, 2001
6
Integrity and Security
641 Need for Triggers
Triggers are useful mechanisms for alerting humans or for starting certain tasks automatically when certain conditions are met As an illustration, suppose that, instead of allowing negative account balances, the bank deals with overdrafts by setting the account balance to zero, and creating a loan in the amount of the overdraft The bank gives this loan a loan number identical to the account number of the overdrawn account For this example, the condition for executing the trigger is an update to the account relation that results in a negative balance value Suppose that Jones withdrawal of some money from an account made the account balance negative Let t denote the account tuple with a negative balance value The actions to be taken are: Insert a new tuple s in the loan relation with s[loan-number] = t[account-number] s[branch-name] = t[branch-name] s[amount] = t[balance] (Note that, since t[balance] is negative, we negate t[balance] to get the loan amount a positive number) Insert a new tuple u in the borrower relation with u[customer -name] = Jones u[loan-number] = t[account-number] Set t[balance] to 0 As another example of the use of triggers, suppose a warehouse wishes to maintain a minimum inventory of each item; when the inventory level of an item falls below the minimum level, an order should be placed automatically This is how the business rule can be implemented by triggers: On an update of the inventory level of an item, the trigger should compare the level with the minimum inventory level for the item, and if the level is at or below the minimum, a new order is added to an orders relation Note that trigger systems cannot usually perform updates outside the database, and hence in the inventory replenishment example, we cannot use a trigger to directly place an order in the external world Instead, we add an order to the orders relation as in the inventory example We must create a separate permanently running system process that periodically scans the orders relation and places orders This system process would also note which tuples in the orders relation have been processed and when each order was placed The process would also track deliveries of orders, and alert managers in case of exceptional conditions such as delays in deliveries
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