framework/db/QueryBuilder.php
public function buildColumns($columns)
    {
        if (!is_array($columns)) {
            if (strpos($columns, '(') !== false) {
                return $columns;
            }

            $rawColumns = $columns;
            $columns = preg_split('/\s*,\s*/', $columns, -1, PREG_SPLIT_NO_EMPTY);
            if ($columns === false) {
                throw new InvalidArgumentException("$rawColumns is not valid columns.");
            }
        }
        foreach ($columns as $i => $column) {
            if ($column instanceof ExpressionInterface) {
                $columns[$i] = $this->buildExpression($column);
            } elseif (strpos($column, '(') === false) {
                $columns[$i] = $this->db->quoteColumnName($column);
            }
        }

        return implode(', ', $columns);
    }
public function buildColumns($columns)
    {
        if (!is_array($columns)) {
            if (strpos($columns, '(') !== false) {
                return $columns;
            }

            $rawColumns = $columns;
            $columns = preg_split('/\s*,\s*/', $columns, -1, PREG_SPLIT_NO_EMPTY);
            if ($columns === false) {
                throw new InvalidArgumentException("$rawColumns is not valid columns.");
            }
        }
        foreach ($columns as $i => $column) {
            if ($column instanceof ExpressionInterface) {
                $columns[$i] = $this->buildExpression($column);
            } elseif (strpos($column, '(') === false) {
                $columns[$i] = $this->db->quoteColumnName($column);
            }
        }

        return implode(', ', $columns);
    }
Processes columns and properly quotes them if necessary.
It will join all columns into a string with comma as separators.
@param string|array $columns the columns to be processed
@return string the processing result