You mean like this:
```
create table t
(
ID integer primary key,
GroupID integer not null,
Ack integer not null check (Ack in (0,1))
);
create unique index t_Deleteable_ID on t (ID, GroupID) where Ack != 1;
create index t_GroupID on t (GroupID) where Ack != 1;
create trigger t_bi_prune before insert on t
begin
delete from t
where ID = (
select ID
from t as o
where (select count(ID) from t where GroupID == o.GroupID and Ack != 1) > 1
and Ack != 1
order by ID
limit 1
)
and (
select count(ID)
from t
) >= 6;
end;
insert into t values (1, 100, 0), (2, 200, 1), (3, 200, 0), (4, 300, 0), (5, 300, 0), (6, 400, 0), (7, 400, 0);
```
The question is that if there is the following pattern:
1 100 1
2 100 0
3 100 0
which row do you want to delete, row 2 or row 3?
And what about the following:
1 100 0
2 100 1
3 100 0
which row should be deleted, row 1 or row 3?
And also for:
1 100 0
2 100 0
3 100 1
which row should be deleted, row 1 or row 2?