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When describing trigger execution contexts, you discussed how one trigger can cause another trigger to be invoked. Is there a point at which multiple triggers can become a problem if too many are invoked Problems can arise when multiple triggers are invoked and they cause a cascading effect from one table to the next. For example, an attempt to update one table might invoke a trigger that updates another table. That update, in turn, might invoke another trigger that modifies data in yet another table. This process can continue on as one trigger after the next is invoked, creating undesirable results and unplanned data modifications. The condition can be made even worse if a loop is created in which a trigger causes a data modification on a table for which another trigger has fired. For example, a data modification on one table might invoke a trigger that causes a second modification. That modification might invoke another trigger, which in turn invokes another trigger, which invokes yet another trigger. The last trigger might then modify data in the original table, causing the first trigger to fire again, repeating the process over and over until the system fails or an implementationspecific process ends the loop. The best way to prevent unwanted modifications or trigger loops is through careful planning in the database design. Triggers should not be implemented unless you re sure of their impact. In addition to careful planning, you should look to the SQL implementation to determine what sorts of safety nets might be in place to prevent trigger looping or unwanted cascading. For example, some implementations allow you to control whether cascading triggers are allowed, and some limit the number of cascading triggers that can fire. Make sure that you read your product s documentation before creating multiple triggers in your database. Earlier, you mentioned that SQL allows you to define multiple triggers on a table. How are triggers processed if multiple triggers are invoked In SQL, processing of multiple triggers is a concern only if the triggers are defined to fire at the same time (BEFORE or AFTER) and if they re the same type of trigger (INSERT, UPDATE, or DELETE). For example, a multiple trigger scenario would exist if two or more triggers are defined (on the same table) with the AFTER UPDATE keywords. If this condition exists, then the triggers are invoked in the order in which they were defined. Let s take a look at an example to show you what I mean. If you create Trigger1 and then create Trigger2 and then create Trigger3, Trigger1 is invoked first, then Trigger2, and then Trigger3. The problem with this is that SQL does not define any way in which you can change that order. For example, if you decide that you want Trigger3 invoked before Trigger1, your only option based on the SQL standard is to delete Trigger1 and Trigger2 from the schema and then recreate the triggers in the order you want them invoked. Because you did not delete Trigger3, it will move into the top spot and be the first to be invoked because it will then be seen as the first to have been created.
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The final type of trigger that we ll look at is the delete trigger. As you would expect, the delete trigger is invoked when a DELETE statement is executed against the subject table, and as with other triggers, the triggered SQL statements are executed and an action is taken. Now let s take a look at an example that uses the CD_STOCK table and CD_OUT table, as shown in Figure 14-4. Suppose you want to create a trigger on the CD_STOCK table. You want the trigger to insert the deleted values into the CD_OUT table. The following CREATE TRIGGER statement uses a REFERENCING clause to allow the triggered SQL statement to know which data to insert into the CD_OUT table:
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CREATE TRIGGER INSERT_CD_OUT AFTER DELETE ON CD_STOCK REFERENCING OLD ROW AS Old FOR EACH ROW INSERT INTO CD_OUT VALUES ( Old.CD_NAME, Old.CD_TYPE );
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In this statement, you are creating a trigger named INSERT_CD_OUT. The statement is defined with the AFTER DELETE keywords, meaning that the old values are inserted into the CD_OUT table after they have been deleted from the CD_STOCK table. The ON clause identifies the CD_STOCK table as the subject table. Following the ON clause is the REFERENCING clause. The REFERENCING clause uses the OLD ROW option to assign a correlation name of Old. Remember that you can use only the OLD ROW and OLD TABLE options in the REFERENCING clause of a delete trigger definition. This is because there is no new data, only the old data that s being deleted.
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