使用codeigniter在数据库中存储数组和更新记录

I am storing dynamic array as well as update the "max_quantity" table in database. The problem is that when update the record than it will sum all the quantity values and subtract it from the 1st value of "max quantity table". But I want that array of quantity is subtracted its own quantity value which is present in the database.I am storing dynamic array as well as update th