public static SelectExpression RemoveOrderExpression(this SelectExpression select, OrderExpression ordering)
 {
     if (select.OrderBy != null && select.OrderBy.Count > 0)
     {
         List <OrderExpression> orderby = new List <OrderExpression>(select.OrderBy);
         orderby.Remove(ordering);
         return(select.SetOrderBy(orderby));
     }
     return(select);
 }
        public static SelectExpression AddOrderExpression(this SelectExpression select, OrderExpression ordering)
        {
            List <OrderExpression> orderby = new List <OrderExpression>();

            if (select.OrderBy != null)
            {
                orderby.AddRange(select.OrderBy);
            }
            orderby.Add(ordering);
            return(select.SetOrderBy(orderby));
        }
        protected override Expression VisitSelect(SelectExpression select)
        {
            // select * from table order by x skip s take t
            // =>
            // select * from (select top s * from (select top s + t from table order by x) order by -x) order by x

            select = (SelectExpression)base.VisitSelect(select);

            if (select.Skip != null && select.Take != null && select.OrderBy.Count > 0)
            {
                var skip         = select.Skip;
                var take         = select.Take;
                var skipPlusTake = PartialEvaluator.Eval(Expression.Add(skip, take));

                select = select.SetTake(skipPlusTake).SetSkip(null);
                select = select.AddRedundantSelect(this.language, new TableAlias());
                select = select.SetTake(take);

                // propogate order-bys to new layer
                select = (SelectExpression)OrderByRewriter.Rewrite(this.language, select);
                var inverted = select.OrderBy.Select(ob => new OrderExpression(
                                                         ob.OrderType == OrderType.Ascending ? OrderType.Descending : OrderType.Ascending,
                                                         ob.Expression
                                                         ));
                select = select.SetOrderBy(inverted);

                select = select.AddRedundantSelect(this.language, new TableAlias());
                select = select.SetTake(Expression.Constant(0)); // temporary
                select = (SelectExpression)OrderByRewriter.Rewrite(this.language, select);
                var reverted = select.OrderBy.Select(ob => new OrderExpression(
                                                         ob.OrderType == OrderType.Ascending ? OrderType.Descending : OrderType.Ascending,
                                                         ob.Expression
                                                         ));
                select = select.SetOrderBy(reverted);
                select = select.SetTake(null);
            }

            return(select);
        }