protected override Expression VisitScalar(ScalarExpression scalar)
 {
     scalar = (ScalarExpression)base.VisitScalar(scalar);
     if (_currentGroupBys != null && _currentFrom != null)
     {
         if (scalar.Query is SelectExpression innerSelect && SourcesAreEqual(_currentFrom, innerSelect.From))
         {
             var groupedColumns = GroupedColumnGatherer.Gather(innerSelect.Where);
             if (groupedColumns?.SequenceEqual(_currentGroupBys) ?? false)
             {
                 var innerAggExpr = (AggregateExpression)innerSelect.Columns[0].Expression;
                 if (innerAggExpr.Argument == null)
                 {
                     return(new AggregateExpression(
                                innerAggExpr.Type,
                                innerAggExpr.AggregateType,
                                null,
                                innerAggExpr.IsDistict
                                ));
                 }
                 else if (innerAggExpr.Argument is ColumnExpression column)
                 {
                     var source = GetSource(_currentFrom, innerSelect.From, column.Alias);
                     return(new AggregateExpression(
                                innerAggExpr.Type,
                                innerAggExpr.AggregateType,
                                new ColumnExpression(column.Type, source.Alias, column.Name),
                                innerAggExpr.IsDistict
                                ));
                 }
             }
         }
     }
     return(scalar);
 }
            public static List <ColumnExpression> Gather(Expression expression)
            {
                var g = new GroupedColumnGatherer();

                g.Visit(expression);
                return(g._columns);
            }