ANTLR PlSqlParser GetColumns

GetColumns.cs

private IEnumerable<string> GetColumns(PlSqlParser.Select_statementContext ssc)
{
    var sl = ssc
        .select_only_statement()
        .subquery()
        .subquery_basic_elements()
        .query_block()
        .selected_list();

    var slel = sl.select_list_elements();

    if (!slel.Any())
    {
        yield return sl.GetChild<TerminalNodeImpl>(0).GetText(); // SELECT *
    }

    foreach (var sle in slel)
    {
        if (sle.column_alias() is { } ca)
        {
            yield return ca.identifier().GetText(); // XXX AS COL_NAME
            continue;
        }

        var ue = sle.expression()
            .logical_expression()
            .unary_logical_expression()
            .multiset_expression()
            .relational_expression()
            .compound_expression()
            .concatenation(0)
            .model_expression()
            .unary_expression();

        if (ue.atom() is { } a)
        {
            yield return a.expressions()
                             ?.expression(0)
                             ?.GetText() // DISTINCT(XX)
                         ?? a.constant()
                             .quoted_string()
                             .Select(qs => qs.variable_name()
                                 .id_expression()
                                 .Last()
                                 .GetText()).FirstOrDefault(); // CONCAT(X,Y) COL_NAME
        }
        else if (ue.case_statement()
            ?.simple_case_statement()
            ?.label_name(0) is { } ln)
        {
            yield return ln.GetText(); // CASE ... WHEN ... THEN ... END COL_NAME
        }
    }
}