What is the purpose of a trigger in SQL?

A trigger in SQL is a stored procedure that automatically executes in response to certain events on a particular table or view.

In more detail, a trigger is a special type of stored procedure that is defined to execute automatically when an INSERT, UPDATE or DELETE operation, also known as DML events, occurs on a specific table or view. Triggers can also be defined to execute on DDL events such as CREATE, ALTER, and DROP, and even database operations like LOGON, LOGOFF, STARTUP, and SHUTDOWN.

Triggers are very useful for maintaining the integrity of the data in the database. They allow the database to enforce certain integrity rules that cannot be enforced by standard integrity checks. For example, a trigger can be used to ensure that a field that is supposed to contain a unique value indeed contains a unique value.

Triggers can also be used to automatically update or calculate a value when a change is made to a related field. For instance, if you have a table that tracks the quantity of items in stock and another table that tracks the quantity of items sold, you could create a trigger that automatically updates the quantity in stock whenever a sale is made.

Moreover, triggers can be used to log changes or events. For example, you could create a trigger that logs every update operation on a specific table, including the old and new values, the time of the change, and the user who made the change. This can be very useful for auditing purposes.

In summary, a trigger in SQL is a powerful tool that allows the database to automatically enforce integrity rules, calculate and update values, and log changes or events. It's important to note, however, that triggers should be used judiciously, as they can significantly impact the performance of the database if not used properly.