IF NOT EXIST Recored选择一些数据

Question

我写了一个查询从tbl中选择最高记录。我要检查，如果没有记录在TBL我的查询返回像（StudentId = 1，高分= 0）假数据

var queryWin = (from T in ((from tbl_ActPoints in dc.tbl_ActPoints 
       select new 
       { 
        tbl_ActPoints.StudentId, 
        tbl_ActPoints.Score 
       })) 
    group T by new 
    { 
     T.StudentId 
    } into g 
    orderby 
     ((System.Int32?)g.Sum(p => p.Score) ?? (System.Int32?)0) descending 
    select new 
    { 
     g.Key.StudentId, 
     HighScore = ((System.Int32?)g.Sum(p => p.Score) ?? (System.Int32?)0) 
    }).Take(1); 

Answer 1

试试这个：

var queryWin = (from T in ((from tbl_ActPoints in dc.tbl_ActPoints 
       select new 
       { 
        tbl_ActPoints.StudentId, 
        tbl_ActPoints.Score 
       })) 
    group T by new 
    { 
     T.StudentId 
    } into g 
    orderby 
     ((System.Int32?)g.Sum(p => p.Score) ?? (System.Int32?)0) descending 
    select new 
    { 
     g.Key.StudentId, 
     HighScore = ((System.Int32?)g.Sum(p => p.Score) ?? (System.Int32?)0) 
    }).Take(1); 

var result = queryWin.FirstOrDefault(); 

if (result == null) 
    result = new { StudentId = 1, HighScore=0 }; 

Answer 2

清理代码位：

var queryWin = (from tbl_ActPoints in dc.tbl_ActPoints 
       group new 
       { 
        tbl_ActPoints.StudentId, 
        tbl_ActPoints.Score 
       } by tbl_ActPoints.StudentId into g 
       orderby 
        (g.Sum(p => p.Score) ?? 0) descending 
       select new StudentHighScore 
       { 
        g.Key.StudentId, 
        HighScore = (g.Sum(p => p.Score) ?? 0) 
       }).FirstOrDefault() 
        ?? new StudentHighScore { StudentID = 1, HighScore = 0}; 

诀窍，原因coolswastik的代码没有工作，因为这两个匿名对象，即使有相同的属性，总是不同的，所以你需要一个名为类：

class StudentHighScore 
{ 
    public int StudentId { get; set; } 
    public int HighScore { get; set; } 
}