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Triggers and Referential Integrity
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Triggers provide an alternative way to implement the referential integrity constraints provided by foreign keys and primary keys. In fact, advocates of the trigger feature point out that the trigger mechanism is more flexible than the strict referential integrity provided by the ANSI/ISO standard. However, opponents of the trigger feature point out that trigger behavior varies greatly from one DBMS to another, particularly in the way transactions are rolled back, how failed triggers are cleaned up, and how locking works during the duration of trigger execution. For example, here is a SQL Server trigger that enforces referential integrity for the OFFICES/SALESREPS relationship and displays a message when an attempted update fails:
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CREATE ON FOR AS TRIGGER REP_UPDATE SALESREPS INSERT, UPDATE IF ((SELECT COUNT(*) FROM OFFICES, INSERTED WHERE OFFICES.OFFICE = INSERTED.REP_OFFICE) = 0) BEGIN PRINT 'Invalid office number specified.' ROLLBACK TRANSACTION END;
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Triggers can also be used to provide extended forms of referential integrity. For example, DB2 initially provided cascaded deletes through its CASCADE delete rule, but did not support cascaded updates if a primary key value is changed. This limitation need not apply to triggers, however. The following SQL Server trigger cascades any update of the OFFICE column in the OFFICES table down into the REP_OFFICE column of the SALESREPS table:
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CREATE ON FOR AS TRIGGER CHANGE_REP_OFFICE OFFICES UPDATE IF UPDATE (OFFICE) BEGIN UPDATE SALESREPS SET SALESREPS.REP_OFFICE = INSERTED.OFFICE FROM SALESREPS, INSERTED, DELETED WHERE SALESREPS.REP_OFFICE = DELETED.OFFICE END;
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As in the previous SQL Server example, the references DELETED.OFFICE and INSERTED.OFFICE in the trigger refer, respectively, to the values of the OFFICE column before and after the UPDATE statement. The trigger definition must be able to differentiate between these before and after values to perform the appropriate search and update actions specified by the trigger.
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Over the last several years, the trigger mechanisms in many commercial DBMS products have expanded significantly. In many commercial implementations, the distinctions between triggers and stored procedures (described in 20) have blurred, so the
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Part III:
Updating Data
action triggered by a single database change may be defined by hundreds of lines of stored procedure programming. The role of triggers has thus evolved beyond the enforcement of data integrity into a programming capability within the database. A complete discussion of triggers is beyond the scope of this book, but even these simple examples show the power of the trigger mechanism. The major advantage of triggers is that business rules can be stored in the database and enforced consistently with each update to the database. This can dramatically reduce the complexity of application programs that access the database. Triggers also have some disadvantages, including these: Database complexity When the rules are moved into the database, setting up the database becomes a more complex task. Users who could reasonably be expected to create small ad hoc applications with SQL will find that the programming logic of triggers makes the task much more difficult. Hidden rules With the rules hidden away inside the database, programs that appear to perform straightforward database updates may, in fact, generate an enormous amount of database activity. The programmer no longer has total control over what happens to the database. Instead, a program-initiated database action may cause other, hidden actions. Hidden performance implications With triggers stored inside the database, the consequences of executing a SQL statement are no longer completely visible to the programmer. In particular, an apparently simple SQL statement could, in concept, trigger a process that involves a sequential scan of a very large database table, which could take a long time to complete. These performance implications of any given SQL statement are invisible to the programmer.
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