php - Decreasing a column value using SET -

i'm trying use codeigniter's database classes decrease value of column. @ moment i'm doing this:

public function deduct_limit($bytes, $ip_address) {     $this->db->where('ip_address', $ip_address);     $this->db->set('limit', 'limit - ' . $bytes, false);     $this->db->update('limits'); } 

however, codeigniter throws error message:

you have error in sql syntax; check manual corresponds mysql server version right syntax use near 'limit - 418266480 ip_address = '127.0.0.1'' @ line 1

update `limits`      set `limit` = limit - 418266480  `ip_address` = '127.0.0.1' 

as far know sql correct, i've google'd , looked on , follow syntax. can not use other +1 or -1?

you need escape reserved words in mysql limit backticks

update limits set `limit` = `limit` - 418266480 ip_address = '127.0.0.1'