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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 DB2 and the ANSI/ISO standard. For example, here is a trigger that enforces referential integrity for the OFFICES/SALESREPS relationship and displays a message when an attempted update fails: 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: 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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Trigger Advantages and Disadvantages
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A complete discussion of triggers is beyond the scope of this book, but even these simple examples shows 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 would take a long time to complete. These performance implications of any given SQL statement are invisible to the programmer.
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Triggers and the SQL Standard
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Triggers were one of the most widely praised and publicized features of Sybase SQL Server when it was first introduced, and they have since found their way into many commercial SQL products. Although the SQL2 standard provided an opportunity to standardize the DBMS implementation of triggers, the standards committee included check constraints instead. As the trigger and check-constraint examples in the preceding sections show, check constraints can be effectively used to limit the data that can be added to a table or modified in a table. However, unlike triggers, they lack the ability to cause an independent action in the database, such as adding a row or changing a data item in another table. The extra capability provided by triggers has led several industry experts to advocate that they be included in a future SQL3 standard. Other experts have argued that triggers are a pollution of the data management function of a database, and that the functions performed by triggers belong in fourth generation languages (4GLs) and other database tools, rather than in the DBMS itself. While the debate continues, DBMS products have experimented with new trigger capabilities that extend beyond the database itself. These "extended trigger" capabilities allow modifications to data in a database to automatically cause actions such as sending mail, alerting a user, or launching another program to perform a task. This makes triggers even more useful and will add to the debate over including them in future official SQL standards. Regardless of the official stance, it appears that triggers will become a more important part of the SQL language over the next several years.
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The SQL language provides a number of features that help to protect the integrity of data stored in a relational database: Required columns can be specified when a table is created, and the DBMS will prevent NULL values in these columns. Data validation is limited to data type checking in standard SQL, but many DBMS products offer other data validation features. Entity integrity constraints ensure that the primary key uniquely identifies each entity represented in the database. Referential integrity constraints ensure that relationships among entities in the database are preserved during database updates. The SQL2 standard and newer implementations provide extensive referential integrity support, including delete and update rules that tell the DBMS how to handle the deletion and modification of rows that are referenced by other rows. Business rules can be enforced by the DBMS through the trigger mechanism popularized by Sybase and SQL Server. Triggers allow the DBMS to take complex actions in response to events such as attempted INSERT, DELETE, or UPDATE statements. Check constraints provide a more limited way to include business rules in the definition of a database and have the DBMS enforce them.
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