Question

How can I create a insert trigger in sqlite? I should check if the number of rows is < 5 = ok. If the number of rows is 5 = delete first row.
Is this way of collecting garbage efficient?

Answer 1

The documentation is quite good: http://www.sqlite.org/lang_createtrigger.html

You should end up with something like

CREATE TRIGGER rowlimit5
  AFTER INSERT ON table
  BEGIN
     DELETE FROM table WHERE ROWID NOT IN (
       SELECT ROWID FROM table ORDER BY ROWID DESC LIMIT 5
     );
  END;