SQLize Online / PHPize Online  /  SQLtest Online

create table test (id int primary key, parent_id int); insert into test values (1, null), (2, null), (3, 1), (4, 1), (5, 2), (6, 2), (7, 12), (8, 21); -- get records with wrong parents select * from test where parent_id is not null and not exists ( select id from test as parents where parents.id = test.parent_id ); -- delete records with wrong parents delete test.* from test left join test as parents on parents.id = test.parent_id where test.parent_id is not null and parents.id is null; select * from test; -- add foreign key to prevent missing parents alter table test add foreign key (parent_id) references test(id);