Linq来自分组的第一个价值-为什么这不起作用？

Question

I asked a question here about how to get specific data from a datatable using Linq and have come up with a solution I thought could do the trick, but it's not giving me what I expected.

Basically, suppose I have the following data to query:

Col_A:     Col_B:     Col_C:    Col_D:    Date1:     Date2:
a1         b1         c1        d1        Dt1_1      Dt2_1
a1         b2         c2        d2        Dt1_1      Dt2_2
a1         b3         c3        d3        Dt1_1      Dt2_3
a1         b4         c4        d4        Dt1_2      Dt2_4
a1         b5         c5        d5        Dt1_2      Dt2_5

And, in terms of dates, assume, for example, "_1" < "_2". So, what I want is, for each value in Col_A , to get one row for each Date1 for the earliest Date2 .

So, given this data, I'd want to get back:

Col_A:     Col_B:     Col_C:    Col_D:    Date1:     Date2:
a1         b1         c1        d1        Dt1_1      Dt2_1
a1         b4         c4        d4        Dt1_2      Dt2_4

In other words, since Date1 changed, we get the data for the earliest Date2 . I hope that makes sense.

And I could not use a complex query to get the data due to it being from a very old DB model that doesn't support many modern grouping functions.

So, what I came up with was:

SQL Query :

SELECT  Col_A, Col_B, Col_C, Col_D, Date1, Date2
FROM MyTable
ORDER BY Col_A, Date1, Date2

This gives me the ordered data and I query that into a DataTable in .Net.

Now, I basically want the first record for each value of Col_A and Date1 combination, so I came up with this Linq statement:

Dim y As DataTable = (From dr As DataRow In OriginalData
    Group dr By k = New With {.Col_A= dr.Field(Of String)("Col_A"), .Date1 = dr.Field(Of Date)("Date1")} Into grp = Group
    Select grp.First).CopyToDataTable

And I thought that would do it, but it returns y having the same number of rows as my original DataTable (in other words, it didn't remove all the duplicates).

As I inspected it closer, it seems that it doesn't do the grouping - It basically sees each row as a new group...*

What am I doing wrong? How can I change this Lync query to give me the correct results?

Also, even though the Linq is written in VB, I'm equally comfortable with C#.

Thanks!!!

Answer 1

Hope this helps:

 public class Test
 {
    public DateTime Date1;
    public DateTime Date2;
    public string Col_A;
 }

...

List<Test> test = new List<Test>();
test.GroupBy(t => t.Col_A).Select(group =>
                          group.OrderBy(e => e.Date2).First())
                          ToList();

Answer 2

This should do what you want - first group by Col_A and Date1 , then sort the groups by Date2 and pick the first item from each group.

originalData.GroupBy(row => new { row.Col_A, row.Date1 })
            .Select(group => group.OrderBy(groupRow => groupRow.Date2)
                                  .First())
            .ToList();