A trigger is a database object that automatically executes a specific action in response to certain database events or changes, such as a data insertion, update or deletion. Triggers are often used in database management systems to maintain data integrity, enforce business rules, or perform custom operations in response to specific events.

Example

Let's say you have a database that stores order information, including the order date, customer ID, and total amount. You want to create a trigger that will update the customer's account balance whenever a new order is inserted into the database.

First, you would write the SQL statement for updating the customer's account balance. For example, you might want to add the total amount of the new order to the customer's current balance:

UPDATE customers
SET account_balance = account_balance + @total_amount
WHERE customer_id = @customer_id;

Next, you would create a trigger that encapsulates this SQL statement:

CREATE TRIGGER UpdateCustomerAccountBalance
ON orders
AFTER INSERT
AS
BEGIN
  DECLARE @customer_id int;
  DECLARE @total_amount decimal(10,2);

  SELECT @customer_id = customer_id, @total_amount = total_amount
  FROM inserted;

  UPDATE customers
  SET account_balance = account_balance + @total_amount
  WHERE customer_id = @customer_id;
END

Now, whenever a new order is inserted into the database, the trigger will automatically execute and update the customer's account balance accordingly.

Basic Use Cases

1. Data integrity: Triggers can be used to enforce data integrity rules, such as preventing the insertion of invalid data or updating related data when a record is changed.

2. Business rules: Triggers can be used to enforce business rules, such as updating customer balances or sending notifications when certain events occur.

3. Audit trails: Triggers can be used to create audit trails or log changes to the database.

4. Custom operations: Triggers can be used to perform custom operations in response to specific events, such as updating a search index or sending a notification.

In summary, triggers are a powerful tool for managing database events and maintaining data integrity in a database management system. They can be used to enforce business rules, perform custom operations, and create audit trails, among other things.