LINQ 连接两个数据表

Question

Hi I have a problem joining two DataTables using LINQ. Tables have columns like this:

table1        table2
ID, name       ID, stock
1, item1       1, 100
2, item2       3, 50
3, item3

I used linq to join like this:

DataTable dtResult = new DataTable();
dtResult.Columns.Add("ID", typeof(string));
dtResult.Columns.Add("name", typeof(string));
dtResult.Columns.Add("stock", typeof(int));

var result = from dataRows1 in table1.AsEnumerable()
             join dataRows2 in table2.AsEnumerable()
             on dataRows1.Field<string>("ID") equals dataRows2.Field<string>("ID")

             select dtResult.LoadDataRow(new object[]
             {
                dataRows1.Field<string>("ID"),
                dataRows1.Field<string>("name"),
                dataRows2.Field<int>("stock"),
              }, false);
result.CopyToDataTable();

Problem is, result only shows IDs which are in the table2.

dtResult
ID, name, stock
1, item1, 100
3, item3, 50

I need to show also the missing items. This is the wanted result:

dtResult
ID, name, stock
1, item1, 100
2, item2, 0  //Prefer if it is "0", otherwise can be left "null"
3, item3, 50

I believe I should do left outer join, but I do not have enough knowledge about linq. Help appreciated. Thank you!

Answer 1

This will let you default to 0 if the row doesn't exist in table2:

var result = from dataRows1 in table1.AsEnumerable()
             join dataRows2 in table2.AsEnumerable()
             on dataRows1.Field<string>("ID") equals dataRows2.Field<string>("ID") into lj
             from r in lj.DefaultIfEmpty()
             select dtResult.LoadDataRow(new object[]
             {
                dataRows1.Field<string>("ID"),
                dataRows1.Field<string>("name"),
                r == null ? 0 : r.Field<int>("stock")
              }, false);

MSDN source

Answer 2

try this:

var result =
from dataRows1 in table1.AsEnumerable()
join dataRows2 in table2.AsEnumerable() on dataRows1.ID  equals dataRows2.ID into ps
from r in ps.DefaultIfEmpty()
select new { C= dataRows1 , r == null ? 0 : dataRows2.Stock};

Answer 3

you simply need use join..into clause like this

var result = from dataRows1 in table1.AsEnumerable()
         join dataRows2 in table2.AsEnumerable()
         on dataRows1.Field<string>("ID") equals dataRows2.Field<string>("ID") into rows
         from row in rows.DefaultIfEmpty()
         select dtResult.LoadDataRow(new object[]
         {
            dataRows1.Field<string>("ID"),
            dataRows1.Field<string>("name"),
            row==null? null : row.Field<int>("stock"),
          }, false);

Answer 4

try this

var result =
    from dataRows1 in table1.AsEnumerable()
    join dataRows2 in table2.AsEnumerable()
    on dataRows1.Field<string>("ID") equals dataRows2.Field<string>("ID") into lj
    from r in lj.DefaultIfEmpty()
    select dtResult.LoadDataRow(new object[]
    {
        dataRows1.Field<string>("ID"),
        dataRows1.Field<string>("name"),
        r == null ? 0 : dataRows2.Field<int>("stock")
    }, false);