How to create a trigger in sql

a trigger lets you write a set of SQL statements that multiple applications can use. It lets you avoid redundant code when multiple programs need to perform the same database operation.
You can use triggers to perform the following actions:

•Create an audit trail of activity in the database. For example, you can track updates to the orders table by updating corroborating information to an audit table.
•Implement a business rule. For example, you can determine when an order exceeds a customer's credit limit and display a message to that effect.
•Derive additional data that is not available within a table or within the database. For example, when an update occurs to the quantity column of the items table, you can calculate the corresponding adjustment to the total_price column.
•Enforce referential integrity. When you delete a customer, for example, you can use a trigger to delete corresponding rows (that is, rows that have the same customer number) in the orders table.