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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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create trigger reorder-trigger after update of amount on inventory referencing old row as orow, new row as nrow for each row when nrowlevel <= (select level from minlevel where minlevelitem = orowitem) and orowlevel > (select level from minlevel where minlevelitem = orowitem) begin insert into orders (select item, amount from reorder where reorderitem = orowitem) end Figure 64 Example of trigger for reordering an item
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minlevel(item, level), which notes the minimum amount of the item to be maintained reorder(item, amount), which notes the amount of the item to be ordered when its level falls below the minimum orders(item, amount), which notes the amount of the item to be ordered We can then use the trigger shown in Figure 64 for reordering the item Note that we have been careful to place an order only when the amount falls from above the minimum level to below the minimum level If we only check that the new value after an update is below the minimum level, we may place an order erroneously when the item has already been reordered Many database systems provide nonstandard trigger implementations, or implement only some of the trigger features For instance, many database systems do not implement the before clause, and the keyword on is used instead of after They may not implement the referencing clause Instead, they may specify transition tables by using the keywords inserted or deleted Figure 65 illustrates how the overdraft trigger would be written in MS-SQLServer Read the user manual for the database system you use for more information about the trigger features it supports
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643 When Not to Use Triggers
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There are many good uses for triggers, such as those we have just seen in Section 642, but some uses are best handled by alternative techniques For example, in the past, system designers used triggers to maintain summary data For instance, they used triggers on insert/delete/update of a employee relation containing salary and dept attributes to maintain the total salary of each department However, many database systems today support materialized views (see Section 351), which provide a much
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create trigger overdraft-trigger on account for update as if nrowbalance < 0 begin insert into borrower (select customer-name, account-number from depositor, inserted where insertedaccount-number = depositoraccount-number) insert into loan values (insertedaccount-number, insertedbranch-name, insertedbalance) update account set balance = 0 from account, inserted where accountaccount-number = insertedaccount-number end Figure 65 Example of trigger in MS-SQL server syntax
easier way to maintain summary data Designers also used triggers extensively for replicating databases; they used triggers on insert/delete/update of each relation to record the changes in relations called change or delta relations A separate process copied over the changes to the replica (copy) of the database, and the system executed the changes on the replica Modern database systems, however, provide built-in facilities for database replication, making triggers unnecessary for replication in most cases In fact, many trigger applications, including our example overdraft trigger, can be substituted by encapsulation features being introduced in SQL:1999 Encapsulation can be used to ensure that updates to the balance attribute of account are done only through a special procedure That procedure would in turn check for negative balance, and carry out the actions of the overdraft trigger Encapsulations can replace the reorder trigger in a similar manner Triggers should be written with great care, since a trigger error detected at run time causes the failure of the insert/delete/update statement that set off the trigger Furthermore, the action of one trigger can set off another trigger In the worst case, this could even lead to an in nite chain of triggering For example, suppose an insert trigger on a relation has an action that causes another (new) insert on the same relation The insert action then triggers yet another insert action, and so on ad in nitum Database systems typically limit the length of such chains of triggers (for example to 16 or 32), and consider longer chains of triggering an error Triggers are occasionally called rules, or active rules, but should not be confused with Datalog rules (see Section 52), which are really view de nitions
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