与多个数据表的左外连接

Question

I have 3 DataTables

DataTable1

Id  Version     URL     Owner
1   1           "xx"    "alice" 
2   1           "yy"    "bob"
3   1           "zz"    "Mike"
4   1           "ww"    "Rob"
5   1           "ww"    "Bick"

DataTable2

Id  Version     DomainID    Region      Type
1   1           aa          asia        1
2   1           bb          europe      2
3   1           cc          africa      1
4   1           dd          aus1        0

DataTable3

Id  Size    FreeSpace
aa  2500    2000
bb  3300    3000
cc  5500    50

Expected Join

Id  Version     URL     Owner       DomainID    Region      Type    Size    Freespace
1   1           "xx"    "alice"     aa          asia        1       2500    2000
2   1           "yy"    "bob"       bb          europe      2       3300    3000
3   1           "zz"    "Mike"      cc          africa      1       5500    50
4   1           "ww"    "sean"      dd          aus1        0       null    null    
5   1           "ww"    "Bick"      null        null        null    null    null

I am doing a Join Operation on these tables using Linq as follows:

// Datatable1 joins with Datatable2 on Id and version (datatable1)  -->  Id and version (datatable2) 
   // Datatable2 joins with Datatable3 on DomainId(datatable2) --> Id(datatable3)


var result = from dataRows1 in DataTable1.AsEnumerable()
                             join dataRows2 in DataTable2.AsEnumerable() on

                             new
                             {
                                 Id = dataRows1.Field<long>("Id"),
                                 Version = dataRows1.Field<long>("version")
                             } equals new
                             {
                                 Id = dataRows2.Field<long>("Id"),
                                 Version = dataRows2.Field<long>("version")
                             }
                              into tempJoin
                              from datarowc in tempJoin.DefaultIfEmpty() 
                             join dataRows3 in DataTable3.AsEnumerable() on                  
         dataRowsc.Field<long>("DomainId") equals dataRows3.Field<long>("Id")
    select new

    {
    datarow1,
    datarowc,
    datarow3
    }

I am getting an exception of datarowc to be null. Not quite sure why datarowc is null here and how to achieve the expected join.

Answer 1

using System.Data;
using System.Linq;

namespace CodeWars
{
    class Program
    {
        static void Main(string[] args)
        {            
            var result = datarows1.AsEnumerable()
                .Select(x => new
                    {
                        Tab1Row = x,
                        Tab2Row = datarows2.AsEnumerable().FirstOrDefault(
                            y => x.Field<int>("Id") == y.Field<int>("Id") &&
                                x.Field<int>("Version") == y.Field<int>("Version")
                        )
                    }
                )
                .Select(x => new
                    {
                        Tab1Row = x.Tab1Row,
                        Tab2Row = x.Tab2Row,
                        Tab3Row = datarows3.AsEnumerable().FirstOrDefault(
                            y => x?.Tab2Row?.Field<string>("DomainId") == y.Field<string>("Id")
                        )
                    }
                );
        }

        static DataTable datarows1 = new DataTable
        {
            Columns = {
                    { "Id", typeof(int) },
                    { "Version", typeof(int) },
                    { "URL", typeof(string) },
                    { "Owner", typeof(string) },
                },
            Rows = {
                    { 1, 1, "xx", "alice" },
                    { 2, 1, "yy", "bob" },
                    { 3, 1, "vv", "mike" },
                    { 4, 1, "ww", "rob" },
                    { 5, 1, "zz", "bick" },
                }
        };

        static DataTable datarows2 = new DataTable
        {
            Columns = {
                    { "Id", typeof(int) },
                    { "Version", typeof(int) },
                    { "DomainID", typeof(string) },
                    { "Region", typeof(string) },
                    { "Type", typeof(int) },
                },
            Rows = {
                    { 1, 1, "aa", "asia", 1 },
                    { 2, 1, "bb", "europe", 2},
                    { 3, 1, "cc", "asia", 1},
                    { 4, 1, "dd", "aus1", 0},
                }
        };

        static DataTable datarows3 = new DataTable
        {
            Columns = {
                    { "Id", typeof(string) },
                    { "Size", typeof(int) },
                    { "FreeSpace", typeof(int) },
                },
            Rows = {
                    { "aa", 2500, 2000 },
                    { "bb", 3300, 3000 },
                    { "cc",5500, 50},
                }
        };

    }
}

.Join() performs inner join, but you want left outer join, so forget about.Join() Code I've provided gives you the result you expect. But maybe you need to add one more Select to form datastructure you need.