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The major disadvantage of triggers is their potential performance impact. If a trigger is set on a particular table, then every database operation that attempts to update that table causes the DBMS to execute the trigger procedure. For a database that requires very high data insertion or update rates, the overhead of this processing can be considerable. This is especially true for bulk load operations, where the data may have already been prechecked for integrity. To deal with this disadvantage, some DBMS brands allow triggers to be selectively enabled and disabled, as appropriate.
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Transact-SQL provides triggers through a CREATE TRIGGER statement in both its Microsoft SQL Server and Sybase Adaptive Server dialects. Here is a Transact-SQL trigger definition for the sample database, which implements the same trigger function as the preceding Oracle PL/SQL example:
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create trigger upd_tgt /* Insert trigger for SALESREPS */ on salesreps for insert as if (@@rowcount = 1) begin update offices set target = target + inserted.quota from offices, inserted where offices.office = inserted.rep_office; end else raiserror 23456
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The first clause names the trigger (UPD_TGT). The second clause is required and identifies the table to which the trigger applies. The third clause is also required and tells which database update operations cause the trigger to be fired. In this case, only an INSERT statement causes the trigger to fire. You can also specify UPDATE or DELETE operations, or a combination of two or three of these operations in a comma-separated list. Transact-SQL restricts triggers so that only one trigger may be defined on a particular table for each of the three data modification operations. The body of the trigger follows the AS keyword. To understand the body of a trigger like this one, you need to understand how Transact-SQL treats the rows in the target table during database modification operations. For purposes of trigger operation, Transact-SQL defines two logical tables whose column structure is identical to the target table on which the trigger is defined. One of these logical tables is named DELETED, and the other is named INSERTED. These
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Database Processing and Stored Procedures
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logical tables are populated with rows from the target table, depending on the data modification statement that caused the trigger to fire, as follows: I DELETE. Each target table row that is deleted by the DELETE statement is placed into the DELETED table. The INSERTED table is empty. I INSERT. Each target table row that is added by the INSERT statement is also placed into the INSERTED table. The DELETED table is empty. I UPDATE. For each target table row that is changed by the UPDATE statement, a copy of the row before any modifications is placed into the DELETED table. A copy of the row after all modifications is placed into the INSERTED table. These two logical tables can be referenced within the body of the trigger, and the data in them can be combined with data from other tables during the trigger s operation. In this Transact-SQL trigger, the trigger body first tests to make sure that only a single row of the SALESREPS table has been inserted, by checking the system variable @@ROWCOUNT. If this is true, then the QUOTA column from the INSERTED logical table is added to the appropriate row of the OFFICES table. The appropriate row is determined by joining the logical table to the OFFICES table based on matching office numbers. Here is a different trigger that detects a different type of data integrity problem. In this case, it checks for an attempt to delete a customer when there are still orders outstanding in the database for that customer. If it detects this situation, the trigger automatically rolls back the entire transaction, including the DELETE statement that fired the trigger:
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create trigger chk_del_cust /* Delete trigger for CUSTOMERS */ on customers for delete as /* Detect any orders for deleted cust # s */ if (select count(*) from orders, deleted where orders.cust = deleted.cust_num) > 0 begin rollback transaction print Cannot delete; still have orders raiserror 31234 end
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