当添加新行并且行数超过您的阈值时,您可以使用触发器删除最旧的行。
行数可以保存在单独的会计表中,以避免每次插入时都有 COUNT。
这是一个完整的例子:
create table bookkeepings (bk_name text primary key, bk_value integer not null);
insert or replace into bookkeepings values ('Max Results', 50);
insert or replace into bookkeepings values ('Qty Results', 0);
create table results
(r_timestamp text primary key default (datetime(current_timestamp)),
result text);
create trigger results_limit_trigger before insert on results"
for each row"
when (select bk_value from bookkeepings where bk_name = 'Qty Results')
>= (select bk_value from bookkeepings where bk_name = 'Max Results')
begin
delete from results
where r_timestamp = (select r_timestamp from results order by r_timestamp limit 1);
end;
create trigger results_count_insert_trigger after insert on results
for each row
begin
update bookkeepings set bk_value = bk_value + 1 where bk_name = 'Qty Results';
end;
create trigger results_count_delete_trigger after delete on results
for each row
begin
update bookkeepings set bk_value = bk_value - 1 where bk_name = 'Qty Results';
end;