Think\Db\Builder::parseField PHP Метод

    protected function parseField($fields, $options = [])
    {
        if ('*' == $fields || empty($fields)) {
            $fieldsStr = '*';
        } elseif (is_array($fields)) {
            // 支持 'field1'=>'field2' 这样的字段别名定义
            $array = [];
            foreach ($fields as $key => $field) {
                if (!is_numeric($key)) {
                    $array[] = $this->parseKey($key, $options) . ' AS ' . $this->parseKey($field, $options);
                } else {
                    $array[] = $this->parseKey($field, $options);
                }
            }
            $fieldsStr = implode(',', $array);
        }
        return $fieldsStr;
    }