What is a trigger in SQL?

SQLMiddle~1 min read

A trigger is a special procedure that runs automatically on a specific event in the database.

It "fires" on INSERT, UPDATE, or DELETE operations against a table.

sql

CREATE TRIGGER log_update
AFTER UPDATE ON users
FOR EACH ROW
INSERT INTO user_logs(user_id, action_time)
VALUES (NEW.id, NOW());

Here the trigger fires after an update (AFTER UPDATE) on the users table and writes the change information into the user_logs table.

So a trigger is the database's automatic reaction to a data change, similar to "an event with an action attached".

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