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call, turns over control to the external procedure, and then receives any return values and parameters. Microsoft SQL Server provides a set of system-defined external stored procedures that provide access to selected operating system capabilities. The XP_SENDMAIL procedure can be used to send electronic mail to users, based on conditions within the DBMS:
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XP_SENDMAIL @RECIPIENTS = Joe , Sam , @MESSAGE = Customer table nearly full ;
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Similarly, the XP_CMDSHELL external procedure can be called to pass commands to the underlying operating system on which SQL Server is operating. Beyond these predefined external procedures, SQL Server allows a user-written external procedure to be stored in a dynamic-linked library (DLL) and called from within SQL Server stored procedures. Informix provides basic access to underlying operating system capabilities with a special SYSTEM statement. In addition, it supports user-written external procedures through its CREATE PROCEDURE statement. Where the statement block comprising the body of an Informix SPL procedure would appear, an EXTERNAL clause specifies the name, location, and language of the externally written procedure. With the procedure defined in this way, it can be called in the same way as native Informix SPL procedures. Newer versions of Oracle (Oracle8 and later) provide the same capability, also via the CREATE PROCEDURE statement. IBM s DB2 database family provides the same set of capabilities.
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As described at the beginning of this chapter, a trigger is a special set of stored procedure code whose activation is caused by modifications to the database contents. Unlike stored procedures created with a CREATE PROCEDURE statement, a trigger is not activated by a CALL or EXECUTE statement. Instead, the trigger is associated with a database table. When the data in the table is changed (by an INSERT, DELETE, or UPDATE statement), the trigger is fired, which means that the DBMS executes the SQL statements that make up the body of the trigger. Triggers can be used to cause automatic updates of information within a database. For example, suppose you wanted to set up the sample database so that any time a new salesperson is inserted into the SALESREPS table, the sales target for the office where the salesperson works is raised by the new salesperson s quota. Here is an Oracle PL/SQL trigger that accomplishes this goal:
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create trigger upd_tgt /* Insert trigger for SALESREPS */ before insert on salesreps for each row when (new.quota is not null) begin update offices set target = target + new.quota; end;
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The CREATE TRIGGER statement is used by most DBMS brands that support triggers to define a new trigger within the database. It assigns a name to the trigger (UPD_TGT for this one) and identifies the table the trigger is associated with (SALESREPS) and the update action(s) on that table that will cause the trigger to be executed (INSERT in this case). The body of this trigger tells the DBMS that for each new row inserted into the table, it should execute the specified UPDATE statement for the OFFICES table. The QUOTA value from the newly inserted SALESREPS row is referred to as NEW.QUOTA within the trigger body.
Advantages and Disadvantages of Triggers
Triggers can be extremely useful as an integral part of a database definition, and they can be used for a variety of different functions, including these: I Auditing changes. A trigger can detect and disallow specific updates and changes that should not be permitted in the database. I Cascaded operations. A trigger can detect an operation within the database (such as deletion of a customer or salesperson) and automatically cascade the impact throughout the database (such as adjusting account balances or sales targets). I Enforce interrelationships. A trigger can enforce more complex interrelationships among the data in a database than those that can be expressed by simple referential integrity constraints or check constraints, such as those that require a sequence of SQL statements or IF THEN ELSE processing. I Stored procedure invocation. A trigger can call one or more stored procedures or even invoke actions outside the DBMS itself through external procedure calls in response to database updates. In each of these cases, a trigger embodies a set of business rules that govern the data in the database and modifications to that data. The rules are embedded in a single place in the database (the trigger definition). As a result, they are uniformly enforced across all applications that access the database. When they need to be changed, they can be changed once with the assurance that the change will be applied uniformly.
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