1 Introduction
Trigger is a powerful function in MySQL database, which can automatically execute some predefined actions when specific database operation events (such as insert, update or delete) occur. This article will introduce the underlying implementation principle of MySQL triggers in detail, and show its specific usage and output results through example analysis.
2. The underlying implementation of the trigger
The underlying implementation of MySQL triggers depends on two system tables, the Triggers table and the Trigger_priv table. The Triggers table is used to store the definition information of all triggers, including the trigger name, database to which it belongs, event type, and trigger timing, etc. The Trigger_priv table is used to control user access to triggers.
When creating a trigger, MySQL will parse and compile the definition statement of the trigger, and store related information in the Triggers table. At the same time, MySQL will also generate a TRIGGER_NAMEfile named , and store the definition of the trigger in this file.
When the event monitored by the trigger occurs, MySQL will read the corresponding trigger definition in the Triggers table and execute the action code in it. During execution, MySQL maintains an event queue for triggers, and events are inserted into the queue for sequential execution.
3. Example analysis
Suppose we have a orderstable called , which contains information about orders. We'd like to automatically update another order_summarytable called , when new orders are inserted, to count the total number of orders for each customer.
First, we need to create a trigger to implement this functionality
CREATE TRIGGER update_order_summary AFTER INSERT ON orders FOR EACH ROW BEGIN UPDATE order_summary SET total_orders = total_orders + 1 WHERE customer_id = NEW.customer_id; END;
The above code creates a trigger named update_order_summarytrigger that ordersexecutes automatically after every insert into the table. You NEW.customer_idcan get the customer ID of the newly inserted row, and then use this ID to update the field order_summaryof the table total_orders.
Next, we perform an insert operation to ordersinsert an order record into the table:
INSERT INTO orders (customer_id, order_date, total_amount) VALUES (1001, '2023-07-01', 99.9);
After the above code is executed, the trigger will be activated and
order_summarythe total number of orders for the corresponding customer in the table will be updated. We can queryorder_summarythe table to verify the result:
SELECT * FROM order_summary;
The output is as follows:
| customer_id | total_orders |
| 1001 | 1 |
It can be seen that the trigger successfully updated order_summarythe data in the table.
4. Summary
Through the introduction of this article, we understand the underlying implementation principles of MySQL triggers. MySQL uses the Triggers table and the Trigger_priv table to store trigger definitions and access permissions. When the trigger is activated, MySQL will parse and execute the action code of the trigger.
This article also shows the specific usage and output results of MySQL triggers through an example analysis. Triggers can automatically execute predefined actions when database operation events occur, providing developers with more powerful data processing capabilities.
In a word, MySQL trigger is a very useful and powerful function. Proper use of triggers can simplify database operations and improve efficiency. Hope this article helps you understand the underlying implementation of MySQL triggers.