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
  constraint   sameuser check (owner = substring_index(user(), '@', 1)),
  PRIMARY KEY (id)
);

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

Function or expression 'user()' cannot be used in the CHECK clause of `sameuser`

INSERT INTO `mytable` VALUES (1,'created by root'),(4,'created by helen'),(6,'created by helen1'),(7,'123');
Table 'db_844849085.mytable' doesn't exist

select * from mytable
Table 'db_844849085.mytable' doesn't exist

INSERT INTO `mytable` VALUES (1,'created by root', 'somebodyelse'),
You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '' at line 1

select user()


user()


u_844849085@localhost