What does a row-level trigger do?

SQLMiddle~1 min read

A row-level trigger is a trigger that fires for every row affected by an INSERT, UPDATE, or DELETE operation.

So if a query changes 5 rows, the trigger runs 5 times, once per row.

Inside such a trigger, special pseudo-records are available:

NEW, the new data (after the change or insert),
OLD, the old data (before the change or delete).

Example

sql

CREATE TRIGGER check_age
BEFORE INSERT ON users
FOR EACH ROW
BEGIN
  IF NEW.age < 18 THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Age must be 18+';
  END IF;
END;

Here the trigger checks every row being inserted and blocks adding a user under 18.

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What does a row-level trigger do?

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