db triggers

Author: mayankSystems

src/main/java/dev/mayankg/db_concepts/mysql/exc07/db_triggers.sql

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+-- MySQL Triggers Basic Syntax
+DELIMITER $$
+
+CREATE TRIGGER trigger_name
+     trigger_time trigger_event ON table_name FOR EACH ROW
+     BEGIN
+     END;
+$$
+
+DELIMITER ;
+
+-- show all triggers
+SHOW TRIGGERS;
+
+-- drop trigger
+DROP TRIGGER IF EXISTS trigger_name;
+
+
+-- MySQL Trigger Example1
+DELIMITER $$
+
+CREATE TRIGGER must_be_adult
+     BEFORE INSERT ON users FOR EACH ROW
+     BEGIN
+          IF NEW.age < 18
+          THEN
+              SIGNAL SQLSTATE '45000'
+                    SET MESSAGE_TEXT = 'Must be an adult!';
+          END IF;
+     END;
+$$
+
+DELIMITER ;
+
+
+-- MySQL Trigger Example2
+DELIMITER $$
+
+CREATE TRIGGER example_cannot_follow_self
+     BEFORE INSERT ON follows FOR EACH ROW
+     BEGIN
+          IF NEW.follower_id = NEW.followee_id
+          THEN
+               SIGNAL SQLSTATE '45000'
+                    SET MESSAGE_TEXT = 'Cannot follow yourself, silly';
+          END IF;
+     END;
+$$
+
+DELIMITER ;
+
+-- MySQL Trigger Example3
+DELIMITER $$
+
+CREATE TRIGGER create_unfollow
+    AFTER DELETE ON follows FOR EACH ROW
+BEGIN
+    INSERT INTO unfollows
+    SET follower_id = OLD.follower_id,
+        followee_id = OLD.followee_id;
+END$$
+
+DELIMITER ;
+
+