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to allow inserting and updating exactly the desired data.
These features provide a lot of power and flexibility, making My SQL significantly more capable than it otherwise might be.
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?
And that’s when you remember that your table has a for each row after-update trigger that inserts into an audit table, synchronises a mirror table, performs some calculations, updates some other columns, and, probably, pauses to smoke a cigar too. To implement it you write an after-insert trigger on the SALES table. If, for whatever reason, your transaction is rolled back after you’ve inserted 100 rows into SALES, those rows will be erased – but what about the emails? It’s almost as if they’re the apple tree in the middle of Eden, a trap placed there to tempt us to sin. I use them to maintain modified by and modified date columns. So take off that I ♥ DATABASE TRIGGERS t-shirt; let’s set fire to it.
Any data that violates any unique index will cause the same problem. For example, I might export some data to a spreadsheet, send it to a client, and the client might update or add some data and return the spreadsheet to me.
That’s a terrible way to update data, but for various reasons, I’m sure many readers have found themselves in a similar situation.
OLD and NEW references may only be used in triggers on events for which they are relevant, as follows: If a WHEN clause is supplied, the SQL statements specified are only executed if the WHEN clause is true.
The issue I was highlighting is that because triggers fire automatically, they can sometimes seem like hidden anarchists lobbing Molotov cocktails into your code from the shadows.
Imagine you have a huge table – 1 million rows – and you need to update a column. If you have triggers on the table, not even Nostradamus knows how many records across the database were really updated. Triggers are also a convenient hiding place for bugs. Tom Kyte points out another reason to be wary of triggers. And it’s not just utl_mail; the same thing would happen with utl_file, utl_http, and many other utl_ functions.
A trigger may be specified to fire whenever a DELETE, INSERT, or UPDATE of a particular database table occurs, or whenever an UPDATE occurs on on one or more specified columns of a table.
At this time SQLite supports only FOR EACH ROW triggers, not FOR EACH STATEMENT triggers.
We can create a trigger to update the 'product_price_history' table when the price of the product is updated in the 'product' table.