public function addWhere($field, $value, $op = 'AND')
    {
        if (!isset($this->sql['WHERE'])) {
            $this->sql['WHERE'] = [];
        }

        if (isset($this->sql['WHERE'][$op])) {
            $this->sql['WHERE'][$op][$field] = $value;
        } else {
            $this->sql['WHERE'][$field] = $value;
            $this->sql['WHERE'][$op] = $this->sql['WHERE'];
        }
    }