presto-parser/src/main/java/io/prestosql/sql/parser/AstBuilder.java/AstBuilder/visitSortItem
class AstBuilder:    @Override
    public Node visitSortItem(SqlBaseParser.SortItemContext context)
    {
        return new SortItem(
                getLocation(context),
                (Expression) visit(context.expression()),
                Optional.ofNullable(context.ordering)
                        .map(AstBuilder::getOrderingType)
                        .orElse(SortItem.Ordering.ASCENDING),
                Optional.ofNullable(context.nullOrdering)
                        .map(AstBuilder::getNullOrderingType)
                        .orElse(SortItem.NullOrdering.UNDEFINED));
    }