how to write a query?

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C#

SQL

LINQ

List<OrderItem> store = new List<OrderItem>();
            IQueryable<OrderItem> in_store = datas.Where(c => c.OrderTypeId == 1).Select(c => c.OrderItems).SelectMany(a => a.Where(c => !c.isdeleted), (a, b) => b);
foreach (OrderItem it in in_store)
            {
       var add = store.SingleOrDefault(c => c.ContragentId == it.ContragentId && c.GoodTypeId == it.GoodTypeId && c.GoodCost == it.GoodCost && c.Order.StoreId == it.Order.StoreId);
                if (add != null) add.GoodCount += it.GoodCount;
                else store.Add(it);
            }

how to rewrite this code in one linq query?
the store variable in result must be iqueryable
all operations does not need to implement ienumerable only iqueryable