[SQL-519] What is a trigger in SQL

Author: Stefanos Georgakis

data-manipulation/triggers/triggers-mysql.sql

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+CREATE TABLE IF NOT EXISTS Student_Audit_Log (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  student_id INT,
+  action VARCHAR(255) NOT NULL,
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+DROP TRIGGER IF EXISTS after_student_insert;
+DROP TRIGGER IF EXISTS after_student_update;
+DROP TRIGGER IF EXISTS after_student_delete;
+
+DELIMITER //
+CREATE TRIGGER after_student_insert
+    AFTER INSERT ON Student
+    FOR EACH ROW
+BEGIN
+    INSERT INTO Student_Audit_Log(student_id, action)
+    VALUES (NEW.id, 'INSERT');
+END//
+
+CREATE TRIGGER after_student_update
+    AFTER UPDATE ON Student
+    FOR EACH ROW
+BEGIN
+    INSERT INTO Student_Audit_Log(student_id, action)
+    VALUES (NEW.id, 'UPDATE');
+END//
+
+CREATE TRIGGER after_student_delete
+    AFTER DELETE ON Student
+    FOR EACH ROW
+BEGIN
+    INSERT INTO Student_Audit_Log(student_id, action)
+    VALUES (OLD.id, 'DELETE');
+END//
+DELIMITER ;

data-manipulation/triggers/triggers-pg.sql

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+CREATE TABLE IF NOT EXISTS Student_Audit_Log (
+  id SERIAL PRIMARY KEY,
+  student_id INT,
+  action VARCHAR(255) NOT NULL,
+  created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE OR REPLACE FUNCTION log_student_changes()
+RETURNS TRIGGER AS $$
+BEGIN
+    IF TG_OP = 'INSERT' THEN
+        INSERT INTO Student_Audit_Log(student_id, action)
+        VALUES (NEW.id, 'INSERT');
+        RETURN NEW;
+    ELSIF TG_OP = 'UPDATE' THEN
+        INSERT INTO Student_Audit_Log(student_id, action)
+        VALUES (NEW.id, 'UPDATE');
+        RETURN NEW;
+    ELSIF TG_OP = 'DELETE' THEN
+        INSERT INTO Student_Audit_Log(student_id, action)
+        VALUES (OLD.id, 'DELETE');
+    RETURN OLD;
+END IF;
+RETURN NULL;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER student_audit_trigger
+    AFTER INSERT OR UPDATE OR DELETE ON Student
+    FOR EACH ROW EXECUTE FUNCTION log_student_changes();
+
+CREATE OR REPLACE FUNCTION log_bulk_deletion_student()
+RETURNS TRIGGER AS $$
+BEGIN
+INSERT INTO Student_Audit_Log (action)
+VALUES ('BULK_DELETION');
+RETURN NULL;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER log_bulk_deletion_student_trigger
+    AFTER DELETE ON Student
+    FOR EACH STATEMENT EXECUTE FUNCTION log_bulk_deletion_student();
+
+CREATE OR REPLACE VIEW Department_View AS
+SELECT * FROM Department;
+
+CREATE OR REPLACE FUNCTION prevent_department_delete()
+RETURNS TRIGGER AS $$
+BEGIN
+RETURN NULL;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER prevent_department_deletion_trigger
+    INSTEAD OF DELETE ON Department_View
+    FOR EACH ROW
+    EXECUTE FUNCTION prevent_department_delete();

data-manipulation/triggers/triggers-sqlserver.sql

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+IF NOT EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[Student_audit_log]') AND type in (N'U'))
+BEGIN
+  CREATE TABLE Student_Audit_Log (
+    id INT IDENTITY(1,1) PRIMARY KEY,
+    student_id INT,
+    action VARCHAR(255) NOT NULL,
+    created_at DATETIME DEFAULT GETDATE()
+);
+END
+
+-- Drop existing triggers if they exist
+IF EXISTS (SELECT * FROM sys.triggers WHERE name = 'trg_student_audit')
+DROP TRIGGER trg_student_audit;
+GO
+
+CREATE TRIGGER trg_student_audit
+    ON Student
+    AFTER INSERT, UPDATE, DELETE
+    AS
+BEGIN
+    SET NOCOUNT ON;
+
+
+    IF EXISTS (SELECT * FROM inserted) AND NOT EXISTS (SELECT * FROM deleted)
+    BEGIN
+        INSERT INTO Student_Audit_Log (student_id, action)
+        SELECT id, 'INSERT'
+        FROM inserted;
+    END
+
+    IF EXISTS (SELECT * FROM inserted) AND EXISTS (SELECT * FROM deleted)
+    BEGIN
+        INSERT INTO Student_Audit_Log (student_id, action)
+        SELECT id, 'UPDATE'
+        FROM inserted;
+    END
+
+    IF EXISTS (SELECT * FROM deleted) AND NOT EXISTS (SELECT * FROM inserted)
+    BEGIN
+        INSERT INTO Student_Audit_Log (student_id, action)
+        SELECT id, 'DELETE'
+        FROM deleted;
+    END
+END
+GO;
+
+CREATE TRIGGER prevent_department_deletion_trigger
+    ON Department
+    INSTEAD OF DELETE
+AS
+BEGIN
+  RETURN;
+END;