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For example, when a new record (representing a new worker) is added to the employees table, new records should also be created in the tables of the taxes, vacations and salaries.
Triggers can also be used to log historical data, for example to keep track of employees' previous salaries.
What's more, we can wrap up the whole transformation process into this one Oracle MERGE command, referencing the external table and the table function in the one command as the source for the MERGED Oracle data.
file,10) full (contracts_file) */ * from contracts_file ))) f on d.contract_id = f.contract_id when matched then update set desc = f.desc, init_val_loc_curr = f.init_val_loc_curr, init_val_adj_amt = f.init_val_adj_amt when not matched then insert values ( f.contract_id, f.desc, f.init_val_loc_curr, f.init_val_adj_amt); In Oracle Database 10g, the MERGE statement has been extended to cover a larger variety of complex and conditional data transformations, allowing faster loading of large volumes of data.
Database-level triggers can help enforce multi-table constraints, or emulate materialized views.
If an exception is raised in a TRANSACTION COMMIT trigger, the changes made by the trigger so far are rolled back and the client application is notified, but the transaction remains active as if COMMIT had never been requested; the client application can continue to make changes and re-request COMMIT.
It’s a huge task, but your table is properly indexed so you’re not too worried. Two hours later you’re sitting there scratching your head. Imagine you write a simple statement to update 10 records and run it. When your code errors, it is natural to go over the code you have just written looking for fractures in the logic; no one ever remembers to look in the triggers that may be firing in silence. In fact, in researching this article, I learned that I was wrong when I wrote that simple triggers to populate primary key columns from sequences are pretty uncontroversial. They recommend that you build APIs to interact with your tables, and populate your primary key columns that way. Imagine you maintain a database for a business that sends a £5 discount code to every customer who makes a purchase. The customers are probably blowing your cash right now. And it is a popular practice to write audit records using autonomous transactions fired from within triggers. If your main process is rolled back, you’ll be left with your audit records. At this point you’re probably asking the sensible question: if triggers are evil, why do they exist?: OLD.salary); dbms_output.put_line('New salary: '