delimiter //

CREATE TRIGGER table1_before_insert BEFORE INSERT ON table1 FOR EACH ROW
BEGIN
DECLARE last_id1 INT;
IF NEW.id1 IS NULL THEN
BEGIN
SELECT MAX(id1) INTO last_id1 FROM table1;
IF last_id1 IS NULL THEN
SET NEW.id1 = 101;
ELSE
SET NEW.id1 = last_id1 + 5;
END IF;
UPDATE table2 SET id2=LAST_INSERT_ID(NEW.id1);
END;
END IF;
END;//

delimiter ;