The Trigger Function Special Variables in VS .NET

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Table 9-2 The Trigger Function Special Variables
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Creating a Trigger Function
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Just as with stored procedures, you can use the pgAdmin III program to create trigger functions The trigger functions are grouped together within each schema The individual triggers are shown as objects under the table object they are defined for To create a trigger function, right-click the Trigger Functions object and select New Trigger Function The New Trigger Function window appears, shown in Figure 9-3 Just as with the stored procedure, the New Trigger Function window provides textboxes and check boxes to allow you to define the trigger function features Remember to set the Language textbox to plpgsql For a trigger function that updates table records, you must define it as a VOLATILE function On the Parameter tab, notice that you are not allowed to define arguments The Definition textbox is available to enter the function code As an example, let s revisit the store database objects created in 4 Suppose you now need to create a trigger that automatically updates the inventory count for a product in the Product table every time an order is entered into the Order table This is a perfect example of showing how an event on one table can alter data in a totally different table
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Figure 9-3 The New Trigger Function window
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In the Trigger Function object within the store schema in the test database created in 4, create a new trigger function called update_inventory Set the Owner value to postgres, set the Language to plpgsql, keep the Volatility set to VOLATILE, and check the Security of Definer check box (this allows anyone with access to the Order table to run the trigger, which will modify the Product table) On the Parameter tab, type the following function code:
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DECLARE onhand int4; BEGIN SELECT into onhand "Inventory" from store"Product" where "Product""ProductID" = NEW"ProductID"; IF TG_OP = 'INSERT' THEN update store"Product" set "Inventory" = onhand - NEW"Quantity" where "ProductID" = NEW"ProductID"; END IF; END;
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Stored Procedures and Triggers
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When a new record is inserted into the Order table, the NEW special variable will be set by the database engine to the data values of the inserted record The individual column values are referenced as if the NEW keyword were the table name This function code uses the SELECT INTO command to extract the current inventory value from the Product table that matches the same ProductID value as the new record s value, and places it into the onhand variable Next, the TG_OP special variable is checked to ensure that the event was indeed an INSERT (this is a common practice, so you can use the same trigger function for multiple trigger events) If the event is an INSERT event, the UPDATE command is executed Again, the NEW special variable is used to retrieve the Quantity column data value inserted into the Order table, and to ensure the proper Product record is updated Now that the trigger function is created, you must define a trigger event for the Order table that automatically executes the trigger function whenever someone inserts a new record Under the Order table object is the Triggers object Right-click Triggers and select New Trigger The New Trigger window appears, shown in Figure 9-4
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Figure 9-4 The New Trigger window
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The textboxes and check boxes represent the parameters available for the CREATE TRIGGER function After entering the trigger name, you must check the Row Trigger check box so that the trigger is fired for each record inserted into the Order table Next, select the storeupdate_inventory trigger function from the drop-down box Since we do not want to update the product inventory until after we are sure the order has been entered, click the AFTER radio button, and then check the INSERT check box If the INSERT command for the Order table fails, the trigger will not fire That is all there is Click OK to create the trigger
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