LEFT OUTER JOIN 2数据表

Question

I'm trying to understand how to query, preferably with LINQ, 2 datatables. I would like to do a LEFT OUTER JOIN on them

Datatable1 with col's: [ID] [colA]
DataTable2 with col's: [ID] [ColB] [ColC] ...

Looking to join on that ID.

Can someone please show me an example so I can apply it to the datatables I have? Thank you in advance

Answer 1

This compiles and does what you would expect based on the result set given:

// create the default row to be used when no value found
var defaultRow = DataTable2.NewRow();
defaultRow[0] = 0;
defaultRow[1] = String.Empty;

// the query
var result = from x in DataTable1.AsEnumerable()
    join y in DataTable2.AsEnumerable() on (string)x["ID"] equals (string)y["ID"] 
             into DataGroup
    from row in DataGroup.DefaultIfEmpty<DataRow>(defaultRow)
    select new {a = x["ColA"], b = (string)row["ColB"]};

Answer 2

To get a LEFT OUTER Join

and you should try using @Joanna link.

from x in DataTable1
join y in DataTable2 on x.ID equals y.ID into DataGroup
from item in DataGroup.DefaultIfEmpty(new y.ColB = String.Empty , y.ColC = String.Empty})
select new {x.ColA, item.ColB , item.ColC}

UPDATE

Given what you provide you should look for LINQ-Dataset articles

Here is the code snippet

DataTable DataTable1 = new DataTable();
DataTable DataTable2 = new DataTable();

DataTable1.Columns.Add("ID");
DataTable1.Columns.Add("ColA");
DataTable1.Rows.Add(1, "A");
DataTable1.Rows.Add(2, "B");    

DataTable2.Columns.Add("ID");
DataTable2.Columns.Add("ColB");
DataTable2.Rows.Add(1, "B");

var result = from x in DataTable1.AsEnumerable()
             join y in DataTable2.AsEnumerable() on x["ID"] equals y["ID"] into DataGroup                         
             from item in DataGroup.DefaultIfEmpty()
             select new {
                            ID = x["ID"],
                            ColA = x["ColA"],
                            ColB = item == null ? string.Empty : item["ColB"]
                        };
foreach (var s in result)
    Console.WriteLine("{0}", s);

Answer 3

To get an inner join try this

from x in Datatable1
join y in Datatable2 on x.ID equals y.ID
select new {x.colA, y.ColB, y.ColC }