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我们知道Linq group join 实现 SQL left join ．隨意兩個測試用表：

1. 員工工資表   EmpId varchar(50), //員工 ID DeptId int,//部門ID Salary decimal(18,2) //工資2. 部門表DeptId int, //部門DeptName nvarchar(100)//部門名稱  /// 
    /// Linq GroupJoin  類似于  Sql Left outer join     /// 
            private void btnGroupJoin_Click(object sender, EventArgs e)    {
           DataSet ds = sqlherpler.GetData("SELECT *FROM DepartMent;SELECT * FROM Employee1", null);        DataTable dtEmployee = ds.Tables[1];//員工表        DataTable dtDept = ds.Tables[0]; //部門名稱表        // Linq 表達式形式。 連接得到 部門名稱        var query1 = from item1 in dtEmployee.AsEnumerable() // 員工表作為左表                     join item2 in dtDept.AsEnumerable()// 部門表作為右表                     on item1.Field
   
    ("deptId") equals item2.Field
    
     ("deptId") into g //部門ID相等                     select new {
    EmpId = item1.Field
     
      ("empId"),                                   DeptName =   g.Select (o=>o.Field 
      
       ("deptName")).FirstOrDefault() ?? "Unknow DepartMent",// g.Any() ? g.First().Field 
       
        ("deptName") : "Unknow DepartMent", //得到 部門名稱                                                                 Salary = item1.Field
        
         ("salary") }; dgv1.DataSource = query1.ToList(); //Lambda 函數形式。連接得到每個部門的員工人數，平均工資 var query2 = dtDept.AsEnumerable().GroupJoin(dtEmployee.AsEnumerable(), item1 => item1.Field
         
          ("deptId"), item2 => item2.Field
          
           ("deptId"), (item1, item2) => new { DeptId = item1.Field
           
            ("deptId"), DeptName = item1.Field
            
             ("deptName"), EmployeeNum = item2.Count() , //部門員工人數 SalaryAgv = item2.Any() ? item2.Average(o=>o.Field
             
              ("salary")).ToString ("0.00") : "0.00" //部門員工平均工資 }) ; dgv2.DataSource = query2.ToList(); }
             
            
           
          
         
        
       
      
     
    
   

另，如果是多个字段连接：例

var queryNotExistInLoc = from d in dtDownLoad.AsEnumerable()                                     join l in dtLoc.AsEnumerable()                                     on new {
    k1 = Convert.ToString(d["employmentCode"]), k2 = DateTime.Parse(Convert.ToString(d["time"])) }                                     equals new {
    k1 = Convert.ToString(l["employmentCode"]), k2 = DateTime.Parse(Convert.ToString(l["ftime"])) }                                     into ljoin                                                                         select new {
    d, itemCardType = Convert.ToString (ljoin.Select(o => o["cardType"]).FirstOrDefault() ?? "") };或 var query1 = dtDownLoad.AsEnumerable().GroupJoin(dtLoc.AsEnumerable(),                 item1 => new {
    k1 = Convert.ToString(item1["employmentCode"]), k2 = DateTime.Parse(Convert.ToString(item1["time"])) },                 item2 => new {
    k1 = Convert.ToString(item2["employmentCode"]), k2 = DateTime.Parse(Convert.ToString(item2["ftime"])) },                 (item1, item2) => new {
    item1, itemCardType = Convert.ToString(item2.Select(o => o["cardType"]).FirstOrDefault() ?? "") });            query1.ToList().ForEach(o =>        Console.WriteLine(o.itemCardType)          );

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