在 LINQ 中的 orderby 之后对每组的行进行编号

Question

Say you have columns AppleType, CreationDate and want to order each group of AppleType by CreationDate. Furthermore, you want to create a new column which explicitly ranks the order of the CreationDate per AppleType.

So, the resulting DataSet would have three columns, AppleType, CreationDate, OrderIntroduced.

Is there a LINQ way of doing this? Would I have to actually go through the data programmatically (but not via LINQ), create an array, convert that to a column and add to the DataSet? I have there is a LINQ way of doing this. Please use LINQ non-method syntax if possible.

Answer 1

So are the values actually appearing in the right order? If so, it's easy - but you do need to use method syntax, as the query expression syntax doesn't support the relevant overload:

var queryWithIndex = queryWithoutIndex.Select((x, index) => new
                                              {
                                                  x.AppleType,
                                                  x.CreationDate,
                                                  OrderIntroduced = index + 1,
                                              });

(That's assuming you want OrderIntroduced starting at 1.)

I don't know offhand how you'd then put that back into a DataSet - but do you really need it in a DataSet as opposed to in the strongly-typed sequence?

EDIT: Okay, the requirements are still unclear, but I think you want something like:

var query = dataSource.GroupBy(x => x.AppleType)
     .SelectMany(g => g.OrderBy(x => x.CreationDate)
                       .Select((x, index ) => new {
                               x.AppleType,
                               x.CreationDate,
                               OrderIntroduced = index + 1 }));

Note: The GroupBy and SelectMany calls here can be put in query expression syntax, but I believe it would make it more messy in this case. It's worth being comfortable with both forms.

Answer 2

If you want a pure Linq to Entities/SQL solution you can do something like this:

Modified to handle duplicate CreationDate's

var query = from a in context.AppleGroup
            orderby a.CreationDate
            select new
            {
              AppleType = a.AppleType,
              CreationDate = a.CreationDate,
              OrderIntroduced = (from b in context.AppleGroup
                                 where b.CreationDate < a.CreationDate
                                 select b).Count() + 1
            };