MariaDB 10.5

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CREATE TABLE mytable (
  id           int AUTO_INCREMENT NOT NULL,
  title        varchar(80),
  owner        varchar(30) default substring_index(user(), '@', 1), -- record owner
  PRIMARY KEY (id)
);

INSERT INTO `mytable` VALUES (1,'created by root','root'),(4,'created by helen','helen'),(6,'created by helen1','helen'),(7,'123','lina');
select * from mytable


id
title
owner


1
created by root
root

4
created by helen
helen

6
created by helen1
helen

7
123
lina


create trigger enforce_insert before insert on mytable for each row   if NEW.owner != substring_index(user(), '@', 1) then signal sqlstate '45000' set message_text = 'conflict of owner on insert'; end if
INSERT INTO mytable VALUES (1,'created by root', 'somebodyelse');
conflict of owner on insert

INSERT INTO mytable(title) VALUES (concat('created by ', user()))
create trigger enforce_update before update on mytable for each row  if OLD.owner != substring_index(user(), '@', 1) OR NEW.owner != substring_index(user(), '@', 1) then signal sqlstate '45000' set message_text = 'conflict of owner on update'; end if
update mytable set title=concat(title, ' modified') where title = 'created by root'
conflict of owner on update

update mytable set title=concat(title, ' modified') where owner=substring_index(user(), '@', 1)
create trigger enforce_delete before delete on mytable for each row  if OLD.owner != substring_index(user(), '@', 1) then signal sqlstate '45000' set message_text = 'conflict of owner on delete'; end if
select * from mytable


id
title
owner


1
created by root
root

4
created by helen
helen

6
created by helen1
helen

7
123
lina

8
created by u_376446168@localhost modified
u_376446168


delete from mytable  where title = 'created by root'
conflict of owner on delete

delete from mytable where owner=substring_index(user(), '@', 1)
select * from mytable


id
title
owner


1
created by root
root

4
created by helen
helen

6
created by helen1
helen

7
123
lina
id	title	owner
1	created by root	root
4	created by helen	helen
6	created by helen1	helen
7	123	lina