Application of SQL triggers

The role of SQL triggers and specific code examples

Overview: SQL triggers are a special stored procedure that occurs when the data in the database changes. A piece of code that executes automatically. Triggers can trigger execution when data is inserted (INSERT), updated (UPDATE), or deleted (DELETE). It can be used to implement various complex data constraints, business logic and data consistency control.

Function:

1. Data integrity control: Through triggers, we can define some rules in the database to ensure the integrity and consistency of the data. For example, you can use triggers to limit the value range of a field, check foreign key constraints of related tables, etc.
2. Business logic control: Triggers can help us implement business logic control at the database level. For example, when a record is inserted into the order table, the total amount of the order can be automatically calculated through a trigger and updated to the corresponding field.
3. Data synchronization and replication: When implementing data synchronization and replication between multiple databases, triggers can be used to synchronize updates to the target database when data changes occur in the source database.
4. Logging and auditing: Through triggers, we can implement logging and auditing functions for database operations. That is, when the data changes, the trigger can automatically record the relevant operations to facilitate subsequent query and tracking.

Code example:
The following is a simple example that shows how to create a trigger in MySQL that automatically updates the data of another summary table when a new record is inserted.

1. Create two tables:

CREATE TABLE orders (
  id INT PRIMARY KEY,
  amount DECIMAL(8,2),
  status ENUM('pending', 'complete')
);

CREATE TABLE summary (
  total_amount DECIMAL(8,2)
);

2. Create a trigger to automatically update the total_amount field in the summary table when a new record is inserted in the orders table:

DELIMITER $$
CREATE TRIGGER update_summary AFTER INSERT ON orders
FOR EACH ROW
BEGIN
  UPDATE summary SET total_amount = total_amount + NEW.amount;
END$$
DELIMITER ;

3. Insert a new record to trigger the execution of the trigger:

INSERT INTO orders (id, amount, status) VALUES (1, 100.00, 'complete');

4. Query the summary table to verify the effect of the trigger:

SELECT * FROM summary;

Through the above code example, we can see that when a new record is inserted into the orders table, the trigger automatically performs the operation of updating the summary table, thereby updating the total_amount field in real time.

Summary:
SQL trigger is a powerful tool that can automatically execute a piece of code when the data changes. Through triggers, we can implement functions such as data integrity control, business logic control, data synchronization and replication, logging and auditing. In actual application development, rational use of triggers can improve the security and reliability of the database.

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