Understanding Triggers
Note
Requires MySQL 5 Support for triggers was added to MySQL 5. As such, this chapter is applicable to MySQL 5 or later only.
MySQL statements are executed when needed, as are stored procedures. But what if you wanted a statement (or statements) to be executed automatically when events occur? For example:
Every time a customer is added to a database table, check that the phone number is formatted correctly and that the state abbreviation is in uppercase. Every time a product is ordered, subtract the ordered quantity from the number in stock. Whenever a row is deleted, save a copy in an archive table.
What all these examples have in common is that they need to be processed automatically whenever a table change occurs. And that is exactly what triggers are. A trigger is a MySQL statement (or a group of statements enclosed within BEGIN and END statements) that are automatically executed by MySQL in response to any of these statements:
No other MySQL statements support triggers.
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