How SQLite limits the total number of data in a table

1、 Do it regularly ：

DELETE FROM table where _id NOT IN (SELECT _id from table ORDER BY insertion_date DESC LIMIT 50)

 

2、 Or build a 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;

2.1 explain ：

a. The trigger condition ：

        INSERT ON _table WHEN (select count(*) from _table)>50 

         When inserting a table, check whether the total number of entries is greater than 50, If it is greater than 50 Then start .

b. Delete data

        DELETE FROM _table WHERE _table._id IN  (SELECT _table._id FROM _table ORDER BY _table._id limit (select count(*) -50 from _table ));

        1. Find out what you want to delete id:DELETE FROM _table WHERE _table._id IN  

        2. Delete from the earliest , Save only the latest 50 strip ： Limit the number of queries ,limit (select count(*) -50 from _table ). Yes id Sort , Of course, it can also be sorted by time .

Reference resources ：sqlite How to limit the total number of data in a table _love_android_2011 The column -CSDN Blog