Oracle triggers in Java

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Oh, boy are you in for a treat! Oracle has the most complete set of triggers of any database product in the world. Where SQL Server has statement-level After and Instead-Of triggers, Oracle has 14 different types of triggers, as the following list shows: Before Insert, statement-level Before Insert, row-level Before Update, statement-level Before Update, row-level Before Delete, statement-level Before Delete, row-level After Insert, statement-level After Insert, row-level After Update, statement-level After Update, row-level After Delete, statement-level After Delete, row-level Instead-Of, statement-level Instead-Of, row-level
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What s more, Oracle triggers have more granular control over when they are fired. You can. for example, create an Oracle Update trigger that fires only after one or more specific columns are updated or if specific conditions exist not just every time that an update occurs.
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Remember pseudotables from the section Updating multiple rows by joining the pseudotable, earlier in this chapter Oracle doesn t have pseudotables; instead, it has pseudorows. As you can imagine, a pseudorow is like a single row of a pseudotable. Other similarities exist between pseudotables and pseudorows. Whenever you perform an update in SQL Server, you create two pseudotables for the Update Trigger to use: Deleted and Inserted, where Deleted contains the old pre-update values and Inserted contains the new post-update values. After you perform an update in Oracle, you create two pseudorows for the row-level Update Trigger: Old and New, where Old contains the old pre-update values and New contains the new post-update values. The reason why Oracle uses pseudorows instead of pseudotables is because Oracle permits row-level triggers that fire once for every row affected by the triggering SQL statement. Programming by using pseudorows is actually much easier than programming by using pseudotables, because you have direct access to the values of their columns via bind variables, as in Listing 11-12. Boldfaced code denotes the major changes between Listing 11-7 and Listing 11-12.
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Listing 11-12: Adapting Listing 11-7 as an Oracle row-level Update Trigger
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CREATE TRIGGER tU_OrderItem AFTER Update of Quantity ON OrderItem For Each Row BEGIN /* Put the old ordered quantity back into inventory */ UPDATE InventoryItem SET AvailableToSell = AvailableToSell + :old.Quantity WHERE ItemNumber = :old.ItemNumber /* Remove the new ordered quantity from inventory Remember: the item number may have changed! */ UPDATE InventoryItem SET AvailableToSell = AvailableToSell - :new.Quantity WHERE ItemNumber = :new.ItemNumber END;
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Because of Oracle s powerful capability to define row-level triggers, you can simplify your trigger code even more. The :old and :new values are available via the pseudorows affected by the UPDATE statement, so you can use them directly in your trigger code without declaring
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variables or joining tables. And this trigger fires only if the Quantity column is part of the UPDATE statement and not just every time that the OrderItem table is updated.
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Developers new to Oracle are often stumped by mutating table errors that occur after implementing seemingly well-written triggers, such as the following:
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ORA-04091: table USERNAME.TABLENAME is mutating, trigger/function may not see it.
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Really understanding the nature of what is going wrong whenever these errors occur is important, because if you don t, you spend a lot of time spinning your debugging wheels. A table is said to be mutating if its contents are changing via an INSERT, UPDATE, or DELETE statement or a DELETE CASCADE is performed between related tables. If a trigger attempts to read or modify information in a table while it is mutating, the preceding error occurs. This makes sense, because to try such a thing is like trying to hit a moving target: Until the data has a stable value, it is of no use to the trigger that wants to read or modify it. Take the example of a row-level trigger shown in Listing 11-13, which attempts to read the number of rows in the table to which it is attached.
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Listing 11-13: A row-level trigger unsuccessfully attempting to read from the table to which it is attached
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CREATE OR REPLACE TRIGGER tDAR_CompanyCount AFTER DELETE ON Company FOR EACH ROW DECLARE nRowCount INTEGER; BEGIN SELECT COUNT(*) INTO nRowCount FROM Company; END;
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Now run the following SQL statement against the Company table:
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