LEFT OUTER JOIN 2數據表

Question

我試圖了解如何查詢，最好使用LINQ 2個數據表。 我想對他們做一個左外連接

帶有col的Datatable1：[ID] [colA]
具有col的DataTable2：[ID] [ColB] [ColC] ...

希望加入該ID。

有人可以給我看一個例子，以便將其應用於我擁有的數據表嗎？ 先感謝您

Answer 1

這將根據給定的結果集進行編譯並執行您期望的操作：

// 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

獲得LEFT OUTER Join

並且您應該嘗試使用@Joanna鏈接。

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

根據您提供的內容，您應該查找LINQ-Dataset文章

這是代碼片段

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

要獲得內部連接，請嘗試此

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