how to create mysql trigger and use real life

CREATE TABLE demo(d1 INT);
CREATE TABLE demo2(d2 INT);
CREATE TABLE demo3(d3 INT NOT NULL AUTO_INCREMENT PRIMARY KEY);
CREATE TABLE demo4(
  a4 INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  b4 INT DEFAULT 0
);

delimiter |
//event before and after implement the table
CREATE TRIGGER demotrigger BEFORE INSERT ON demo1
  FOR EACH ROW BEGIN
    INSERT INTO demo2 SET d2 = NEW.d1;
    DELETE FROM demo3 WHERE d3 = NEW.d1;
    UPDATE demo4 SET b4 = b4 + 1 WHERE a4 = NEW.d1;
  END;
|

delimiter ;

INSERT INTO demo3 (d3) VALUES
  (NULL), (NULL), (NULL), (NULL), (NULL),
  (NULL), (NULL), (NULL), (NULL), (NULL);

INSERT INTO demo4 (a4) VALUES
  (0), (0), (0), (0), (0), (0), (0), (0), (0), (0);

Suppose that you insert the following values into table test1 as shown here: