11-6: An incorrectly designed trigger in Java

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Listing 11-6: An incorrectly designed trigger
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CREATE TRIGGER tU_OrderItem ON OrderItem AFTER Update AS BEGIN /* Declare variables for use by the trigger */ DECLARE @iOldQuantity INTEGER, @iNewQuantity INTEGER, @sOldItemNumber VARCHAR(15),
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Listing 11-6 (continued)
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@sNewItemNumber VARCHAR(15) /* Get the old ordered quantity and item number */ SELECT @iOldQuantity = Quantity, @sOldItemNumber = ItemNumber FROM Deleted /* Get the new ordered quantity and item number */ SELECT @iNewQuantity = Quantity, @sNewItemNumber = ItemNumber FROM Inserted /* Update the old item number to put the former ordered quantity back into inventory */ UPDATE InventoryItem SET AvailableToSell = AvailableToSell + @iOldQuantity WHERE ItemNumber = @sOldItemNumber /* Update the new item number (it might have changed) to remove the new ordered quantity from inventory */ UPDATE InventoryItem SET AvailableToSell = AvailableToSell - @iOldQuantity WHERE ItemNumber = @sNewItemNumber END
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This trigger code executes only once and uses only the values from the first row in the Deleted pseudotable, in turn causing only one row in the InventoryItem table to be updated. Consider, however, the trigger code in Listing 11-7.
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Listing 11-7: Modifying all rows affected by the triggering SQL statement by joining a pseudotable
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CREATE TRIGGER tU_OrderItem ON OrderItem
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AFTER Update AS BEGIN /* Put the old ordered quantity back into inventory */ UPDATE InventoryItem SET AvailableToSell = AvailableToSell + d.Quantity FROM Deleted d INNER JOIN InventoryItem ii ON d.ItemNumber = ii.ItemNumber /* Remove the new ordered quantity from inventory Remember: the item number may have changed! */ UPDATE InventoryItem SET AvailableToSell = AvailableToSell - i.Quantity FROM Inserted i INNER JOIN InventoryItem ii ON i.ItemNumber = ii.ItemNumber END
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This trigger code updates all InventoryItem rows that correspond to the OrderItem rows being updated even if the ItemNumber key changes value. By joining the Inserted and Deleted pseudotables with the InventoryItem table, you can perform the necessary calculations to increase and decrease each inventory item by the old and new quantities ordered, respectively. The code is also much less cumbersome if written correctly. In actual practice, you would probably do three things differently. First, you would have three separate triggers to accomplish this task one each for the INSERT, UPDATE, and DELETE events and each would contain the specific logic for adjusting the inventory level according to the database event that occurred. Second, you would place logic in your UPDATE trigger to prevent changing the ItemNumber from its original value. Third, you would consolidate the two UPDATE queries in the trigger into one query joining the InventoryItem table and both pseudotables, as in Listing 11-8.
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Listing 11-8: Modifying the UPDATE trigger to take advantage of a nonmodifiable key
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CREATE TRIGGER tU_OrderItem ON OrderItem AFTER Update AS BEGIN IF (UPDATE(ItemNumber)) BEGIN RAISERROR 50010 You cannot change the Item Number. ROLLBACK TRANSACTION
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Listing 11-8 (continued)
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RETURN END UPDATE InventoryItem SET AvailableToSell = ii.AvailableToSell + d.Quantity - i.Quantity FROM Inserted i INNER JOIN InventoryItem ii ON i.ItemNumber = ii.ItemNumber INNER JOIN Deleted d ON d.ItemNumber = ii.ItemNumber END
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Imagine retrieving a bunch of rows into a separate place in memory and then fetching those rows one at a time, looking at the column values in each row and possibly performing some operation based on the values in each row. This is the life of a cursor. Cursors are a last resort because they are cumbersome, slow, and resource intensive, and they destroy the scalability of your database if used on high-transaction throughput areas of your database. Cursors are often used where they shouldn t be, either as substitutes for good SQL code or to work around poorly designed databases. At times, however, a cursor is the only practical solution to your problem. Say, for example, that you have a stored procedure named sp_ArchiveCall that is used in many places throughout your database, including the Insert trigger shown in Listing 11-9.
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Listing 11-9: Modifying all rows affected by the triggering SQL statement by scrolling over a cursor
CREATE PROCEDURE sp_ArchiveCall ( @SupportCallID Integer ) AS BEGIN UPDATE SupportCall SET Status = 60 WHERE SupportCallID = @SupportCallID END go CREATE TRIGGER tI_SupportCallArchive ON SupportCallArchive AFTER INSERT