108 lines
6.4 KiB
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108 lines
6.4 KiB
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta http-equiv="PICS-Label" content='(PICS-1.1 "http://www.icra.org/ratingsv02.html" l gen true r (cz 1 lz 1 nz 1 oz 1 vz 1) "http://www.rsac.org/ratingsv01.html" l gen true r (n 0 s 0 v 0 l 0) "http://www.classify.org/safesurf/" l gen true r (SS~~000 1))' />
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<meta name="DC.Title" content="BEFORE SQL triggers" />
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<meta name="abstract" content="BEFORE triggers may not modify tables, but they can be used to verify input column values and also to modify column values that are inserted or updated in a table." />
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<meta name="description" content="BEFORE triggers may not modify tables, but they can be used to verify input column values and also to modify column values that are inserted or updated in a table." />
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<meta name="DC.subject" content="trigger, BEFORE trigger, example, CREATE TRIGGER statement, examples" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<title>BEFORE SQL triggers</title>
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</head>
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<body id="rbafybeforesql"><a name="rbafybeforesql"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">BEFORE SQL triggers</h1>
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<div><p>BEFORE triggers may not modify tables, but they can be used to
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verify input column values and also to modify column values that are inserted
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or updated in a table. </p>
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<div class="section"><p>In the following example, the trigger is used to set the fiscal
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quarter for the corporation before inserting the row into the target table.</p>
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<pre><strong>CREATE TABLE</strong> TransactionTable (DateOfTransaction <strong>DATE</strong>, FiscalQuarter <strong>SMALLINT</strong>)
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<strong>CREATE TRIGGER</strong> TransactionBeforeTrigger <strong>BEFORE INSERT ON</strong> TransactionTable
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<strong>REFERENCING NEW AS</strong> new_row
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<strong>FOR EACH ROW MODE DB2ROW</strong>
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<strong>BEGIN</strong>
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<strong>DECLARE</strong> newmonth <strong>SMALLINT</strong>;
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<strong>SET</strong> newmonth = <strong>MONTH</strong>(new_row.DateOfTransaction);
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<strong>IF</strong> newmonth < 4 <strong>THEN</strong>
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<strong>SET</strong> new_row.FiscalQuarter=3;
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<strong>ELSEIF</strong> newmonth < 7 <strong>THEN</strong>
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<strong>SET</strong> new_row.FiscalQuarter=4;
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<strong>ELSEIF</strong> newmonth < 10 <strong>THEN</strong>
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<strong>SET</strong> new_row.FiscalQuarter=1;
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<strong>ELSE</strong>
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<strong>SET</strong> new_row.FiscalQuarter=2;
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<strong>END IF</strong>;
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<strong>END</strong></pre>
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<p>For the SQL insert statement below, the "FiscalQuarter"
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column is set to 2, if the current date is November 14, 2000.</p>
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<pre><strong>INSERT INTO</strong> TransactionTable(DateOfTransaction)
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<strong>VALUES</strong>(<strong>CURRENT DATE</strong>) </pre>
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<p>SQL triggers
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have access to and can use User-defined Distinct Types (UDTs) and stored procedures.
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In the following example, the SQL trigger calls a stored procedure to execute
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some predefined business logic, in this case, to set a column to a predefined
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value for the business.</p>
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<pre><strong>CREATE DISTINCT TYPE</strong> enginesize <strong>AS DECIMAL</strong>(5,2) <strong>WITH COMPARISONS</strong>
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<strong>CREATE DISTINCT TYPE</strong> engineclass <strong>AS VARCHAR</strong>(25) <strong>WITH COMPARISONS</strong>
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<strong>CREATE PROCEDURE</strong> SetEngineClass(<strong>IN</strong> SizeInLiters enginesize,
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<strong>OUT</strong> CLASS engineclass)
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<strong>LANGUAGE SQL CONTAINS SQL</strong>
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<strong>BEGIN</strong>
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<strong>IF</strong> SizeInLiters<2.0 <strong>THEN</strong>
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<strong>SET</strong> CLASS = 'Mouse';
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<strong>ELSEIF</strong> SizeInLiters<3.1 <strong>THEN</strong>
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<strong>SET</strong> CLASS ='Economy Class';
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<strong>ELSEIF</strong> SizeInLiters<4.0 <strong>THEN</strong>
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<strong>SET</strong> CLASS ='Most Common Class';
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<strong>ELSEIF</strong> SizeInLiters<4.6 <strong>THEN</strong>
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<strong>SET</strong> CLASS = 'Getting Expensive';
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<strong>ELSE</strong>
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<strong>SET</strong> CLASS ='Stop Often for Fillups';
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<strong>END IF</strong>;
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<strong>END</strong>
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<strong>CREATE TABLE</strong> EngineRatings (VariousSizes enginesize, ClassRating engineclass)
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<strong>CREATE TRIGGER</strong> SetEngineClassTrigger <strong>BEFORE INSERT ON</strong> EngineRatings
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<strong>REFERENCING NEW AS</strong> new_row
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<strong>FOR EACH ROW MODE DB2ROW</strong>
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<strong>CALL</strong> SetEngineClass(new_row.VariousSizes, new_row.ClassRating)</pre>
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<p>For
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the SQL insert statement below, the "ClassRating" column is set to "Economy
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Class", if the "VariousSizes" column has the value of 3.0.</p>
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<pre><strong>INSERT INTO</strong> EngineRatings(VariousSizes) <strong>VALUES</strong>(3.0)</pre>
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<p>SQL requires all tables, user-defined functions, procedures and user-defined
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types to exist before creating an SQL trigger. In the examples above, all
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of the tables, stored procedures, and user-defined types are defined before
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the trigger is created.</p>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafysqltrig.htm" title="The SQL CREATE TRIGGER statement provides a way for the database management system to actively control, monitor, and manage a group of tables and views whenever an insert, update, or delete operation is performed.">SQL triggers</a></div>
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</div>
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</div>
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</body>
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</html>
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