query.go
func (q *Query) Columns(names ...string) *Query {
	q.headers = append(q.headers, "Columns: "+strings.Join(names, " "))
	q.columns = names
	return q
}
func (q *Query) Columns(names ...string) *Query {
	q.headers = append(q.headers, "Columns: "+strings.Join(names, " "))
	q.columns = names
	return q
}
// Columns selects which columns to retrieve.