1、第一种方式，定期执行
DELETE FROM table where _id NOT IN (SELECT _id from table ORDER BY insertion_date DESC LIMIT 50)
2、第二种方式，建立trigger
CREATE TRIGGER delete_till_50 INSERT ON _table WHEN (select count(*) from _table)>50
BEGIN
DELETE FROM _table WHERE _table._id IN (SELECT _table._id FROM _table ORDER BY _table._id limit (select count(*) -50 from _table ));
END;
