将 SQL 查询转换为 LINQ/点表示法

Posted 2023-03-23

【中文标题】将 SQL 查询转换为 LINQ/点表示法

【英文标题】：convert SQL query to LINQ / dot notation

【发布时间】：2013-07-30 11:26:40

【问题描述】：

我被困在下面的 sql 到 LINQ 查询的转换中。基本上我只想将我的考试分组在我的表格成绩中，然后计算通过、失败、参加次数和通过率的总数

SELECT testID, 
(SELECT COUNT(testID) FROM tblGrade AS b 
    WHERE b.testID= a.testID AND b.Grade < 80) AS 'Failed',
(SELECT COUNT(testID) FROM tblGrade AS b 
    WHERE b.testID= a.testIDAND b.Grade >= 80) AS 'Passed',
--taken = failed + passed, 
--passingrate = (passed / taken) * 100
FROM dbo.tblGrade AS a
GROUP BY testID
ORDER BY testID

编辑： 我的解决方案如下：它有效，但我认为它不是最好的方法，尤其是失败和传递的属性。

var xx1 = _unitOfWork.tblGrade.GetAll().GroupBy(a => new  a.testID);
var xx2 = xx1.Select(b => new
           
             testID= b.Key.testID,
             failed = _unitOfWork.tblGrade.Query(filter: a => a.testID == b.Key.testID).Where(c => c.Grade < 80).Count(),
             passed = _unitOfWork.tblGrade.Query(filter: a => a.testID == b.Key.testID).Where(c => c.Grade >= 80).Count(),
             //taken = failed + passed, 
             //passingrate = (passed / taken) * 100
            ).ToList();

【问题讨论】：

谢谢。我该如何使用它？

【参考方案1】：

Count() 可以将谓词 (Expression<Func<T, bool>>) 作为参数，所以我想你可以这样做

var xx2 = xx1.Select(b => new
           
             testID= b.Key.testID,
             failed = b.Count(x => x.Grade < 80),
             passed = b.Count(x => x.Grade >=80),
             taken = b.Count()
            )
            .Select(b => new 
               testID = b.TestID,
               failed = b.failed,
               passed = b.passed,
               taken = b.taken,
               passingrate = b.taken == 0 ? 0.0 : ((float)b.passed / b.taken) * 100
            ).ToList();

查询语法有（在这种情况下很有用）let 关键字，所以它可能更容易阅读

var xx2 = from b in xx1
          let failed = b.Count(x => x.Grade < 80)
          let passed = b.Count(x => x.Grade >= 80)
          let taken = failed + passed
          select new 
              testID = b.Key.TextID,
              failed = failed,
              passed = passed,
              taken = taken,
              passingrate = taken == 0 ? 0.0 : ((float)passed / taken) * 100
          

【讨论】：

很好的答案。但是sql查询中的“WHERE b.testID = a.testID”怎么样，使用“b.Count（x => x.Grade

@samantha07 分组已经完成。所以你已经按 testID 分组了。计数将是“按 testID”

@samantha07 顺便说一句，我不会像你那样编写“基本 SQL 查询”;)

接受为答案！非常感谢你。虽然想知道为什么通过率总是 0

为什么“b.passed / (b.failed + b.passed)”总是为零？

【参考方案2】：

我会这样做：

  var h = list.GroupBy(a => a.testID)
            .Select(a => 
               int _failed = a.Count(g => g.Grade < 80);
               int _passed = a.Count(g => g.Grade >= 80);
               int _rate = (int)(_passed / (double)a.Count() * 100.0);

               return new 
                 testID = a.Key,
                 failed = _failed,
                 passed = _passed,
                 taken = a.Count(),
                 rate = _rate,
              ;     
            ); 

完整的测试代码：

void Main()

  List<aGrade> list = new List<aGrade>() 
    new aGrade()  Grade = 40, testID = 1 ,
    new aGrade()  Grade = 50, testID = 1 ,
    new aGrade()  Grade = 45, testID = 1 ,
    new aGrade()  Grade = 70, testID = 1 ,
    new aGrade()  Grade = 80, testID = 1 ,
    new aGrade()  Grade = 90, testID = 1 ,
    new aGrade()  Grade = 40, testID = 2 ,
    new aGrade()  Grade = 50, testID = 2 ,
    new aGrade()  Grade = 45, testID = 2 ,
    new aGrade()  Grade = 70, testID = 2 ,
    new aGrade()  Grade = 80, testID = 2 ,
    new aGrade()  Grade = 90, testID = 2 ,
    new aGrade()  Grade = 40, testID = 3 ,
    new aGrade()  Grade = 50, testID = 3 ,
    new aGrade()  Grade = 45, testID = 3 ,
    new aGrade()  Grade = 70, testID = 3 ,
    new aGrade()  Grade = 80, testID = 3 ,
    new aGrade()  Grade = 90, testID = 3 ,
  ;    

  var h = list.GroupBy(a => a.testID)
            .Select(a => 
               int _failed = a.Count(g => g.Grade < 80);
               int _passed = a.Count(g => g.Grade >= 80);
               int _rate = (int)(_passed / (double)a.Count() * 100.0);

               return new 
                 testID = a.Key,
                 failed = _failed,
                 passed = _passed,
                 taken = a.Count(),
                 rate = _rate,
              ;     
            ); 
  h.Dump(); 

// Define other methods and classes here

public class aGrade

    public int Grade   get; set; 
    public int testID  get; set; 
   

NB - 此代码将像在 LinqPad 中一样工作。 (linqpad.com) 我推荐 linqpad 来测试这种类型的代码......让你的工作变得如此简单。试试看。