BEFORE DELETE Trigger

A trigger is SQL that runs automatically on INSERT, UPDATE, or DELETE. CREATE TRIGGER trg BEFORE INSERT ON marks FOR EACH ROW SET NEW.score = IF(NEW.score < 0, 0, NEW.score); clamps negative scores.

BEFORE can change NEW values. AFTER sees the row already written. FOR EACH ROW is the MySQL student form. Don’t hide all business rules in triggers — apps become hard to debug.

OLD is the previous row on UPDATE/DELETE. NEW is the incoming row on INSERT/UPDATE.

Trap — trigger that INSERTs into the same table → loop. Trap: huge logic that belongs in the app.

On the example next to this theory: Create Trigger: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

On the example next to this theory: Show Trigger: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

On the example next to this theory: DROP Trigger: create two demo rows, then SELECT qty >= 2 ordered. Say which labels come back.

On the example next to this theory — Before Insert Trigger — iNSERT adds rows. List columns explicitly for clarity.

On the example next to this theory — After Insert Trigger — iNSERT adds rows. List columns explicitly for clarity.

On the example next to this theory: BEFORE DELETE Trigger — dELETE removes matching rows. TRUNCATE clears a whole table faster (MySQL).

Exam tip

BEFORE INSERT … FOR EACH ROW SET NEW.col = …

BEFORE DELETE Trigger — sample query

CREATE TABLE tags (id INTEGER PRIMARY KEY, label TEXT);
INSERT INTO tags VALUES (1, 'old'), (2, 'keep');
DELETE FROM tags WHERE label = 'old';
SELECT * FROM tags;

BEFORE DELETE Trigger — dELETE removes matching rows. TRUNCATE clears a whole table faster (MySQL).

Short notes

DefTrigger = automatic SQL on INSERT/UPDATE/DELETE.
RuleBEFORE can edit NEW. AFTER cannot. OLD vs NEW.
TrapRecursive triggers. Business logic only in triggers.