SELECT t.test_day_start INTO day_start
FROM test t
WHERE t.test_id = :new.test_id;

create table test_tab(
test_id NUMBER,
test_day_start date);

create or replace view test as select * from test_tab;

create or replace trigger ioi_test
instead of insert on test
for each row
begin
   insert into test_tab (
                           test_id
                         , test_day_start
                        )
      values
              (
                 :new.test_id
               , :new.test_day_start
              )
   ;
end;
/
show errors


create table test2_tab(
test2_id NUMBER,
test_id NUMBER,
test2_day_start date);

create or replace view test2 as select * from test2_tab;

create or replace trigger io_test2
instead of insert on test2
for each row
begin
   insert into test2_tab (
                            test2_id
                          , test_id
                          , test2_day_start
                         )
      select
                 :new.test2_id
               , :new.test_id
               , :new.test2_day_start
      from dual
      where not exists
         (
            select null
            from test
            where test.test_id = :new.test_id
            and test.test_day_start >= :new.test2_day_start
         )
   ;
   if sql%rowcount = 0 then 
raise_application_error(-20999,
'trigger:io_test2:test2_day_start is on or before parent''s day_start');
 end if;
end;
/
show errors


insert into test values (1, sysdate);
insert into test values (2, sysdate+1);
insert into test values (3, sysdate+2);

commit;

select * from test;

insert into test2 values (1, 1, SYSDATE);
insert into test2 values (2, 1, SYSDATE+1.5);
insert into test2 values (3, 1, SYSDATE+1.75);
insert into test2 values (4, 1, SYSDATE+2);
insert into test2 values (5, 2, SYSDATE+4);
insert into test2 values (6, 2, SYSDATE+8);
insert into test2 values (7, 3, SYSDATE+11);

commit;

select * from test2;

--------------- the insert that you want to fail, will fail

insert into test2 values (8, 1, SYSDATE-1);