Defensive Database Programming - Red Gate Software

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Chapter 6: Common Problems with Data IntegrityCREATE TRIGGER dbo.Items_EraseBarcodeChangeLogON dbo.ItemsFOR UPDATEASBEGIN ;DELETE FROM dbo.ItemBarcodeChangeLog ;END ;Listing 6-56: Creating a second FOR UPDATE trigger,Items_EraseBarcodeChangeLog, on table Items.This new trigger creates without any warnings, and we may not realize that we alreadyhave another FOR UPDATE trigger on the same table. We can rerun the script in Listing6-48 and see for ourselves that this new trigger and the previous one do the oppositethings and, as such, they should not coexist. Of course, this is a simplified example, butthe point is that when we have multiple triggers for the same operation on the sametable, then it's a recipe for bugs and data integrity issues.As a defensive technique, before you start coding a trigger, it's well worth running aquick check to find out if any other triggers exist on the target table. To be even safer,after adding a trigger it is a good practice to check for multiple triggers on the same tablefor the same operation. Listing 6-57 shows a script that finds all the tables on whichthere is more than one trigger for the same operation.SELECT OBJECT_NAME(t.parent_id),te.type_descFROM sys.triggers AS tINNER JOIN sys.trigger_events AS teON t.OBJECT_ID = te.OBJECT_IDGROUP BY OBJECT_NAME(t.parent_id),te.type_descHAVING COUNT(*) > 1 ;Listing 6-57: Selecting all the tables on which there is more than one trigger for thesame operation.206
Chapter 6: Common Problems with Data IntegrityMultiple triggers for the same operation, on the same table, not only introduce thechances of conflicting changes, they also introduce the chance of redundant actions, orof unwanted and maybe unknown dependencies on undocumented execution orders.If we want to make sure that there is not more than one trigger for one operation onone table, we can wrap the query in Listing 6-57 in a unit test and have our test harnessverify that it never returns anything.Before moving on to other examples, let us get rid of the triggers we no longer need, asshown in Listing 6-58.DROP TRIGGER dbo.Items_EraseBarcodeChangeLog ;GODROP TRIGGER dbo.Items_LogBarcodeChange ;Listing 6-58: Dropping the triggers.Problems with triggers under snapshot isolation levelsIn Chapter 4, we discussed in detail how triggers can fail when running under snapshotisolation. As we demonstrated there, when our triggers are selecting from other rowsor other tables, in some cases we are implicitly assuming that they will not run undersnapshot isolation level. In such cases, it is necessary to use the READCOMMITTEDLOCKhint to eliminate this assumption.SummaryWe have discussed several cases where incorrect assumptions may lead to compromiseddata integrity, and we have investigated ways to improve the integrity of data by properuse of constraints and triggers.Data integrity logic in the application layer is too easily bypassed. The only completelyrobust way to ensure data integrity is to use a trusted constraint. Even then, we need totest our constraints, and make sure that they handle nullable columns properly.207
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Enforcing Data Integrity in the App
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About the AuthorAlex Kuznetsov has
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IntroductionResilient T-SQL code is
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Chapter 1: Basic Defensive Database
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Defensive Database Programming - Red Gate Software

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