使用set减去更新查询中的列值

Question

I am working on a database column value (int) to subtract and update using set.

I am using $this->db->set() to subtract and update column value in codeigniter.

The problem is when I run the query the column value get change to null.

$this->db->set('app_balance', 'app_balance'. - $app_work_charge.'');
$this->db->where('id', $app_id);
$this->db->update('app');

In my log file it will not showing any error.

More Explanation: if column value is 200 and if I will want to subtract 10 from it then after query execution it will change to null.

Answer 1

Write false at the end of the first line and put - after field name.

$this->db->set('app_balance', 'app_balance-'. $app_work_charge.'',false);
$this->db->where('id', $app_id);
$this->db->update('app');