LINQ左連接與分組並具有轉換這個TSQL

Question

以下TSQL提供的我會如何解決這個問題，SQL的例子。目標是從左表中返回每個OID 1行，其中左表中的記錄計數等於右表中匹配行的計數。

SELECT cs.OID, Count(cs.OID) AS CarCount, Sum(RS.Check) AS RoadCount 
    FROM Cars AS cs 
LEFT JOIN Roads AS RS 
    ON CS.oid = RS.OID 
    AND cs.RID = RS.RID 
    GROUP BY cs.OID 
    HAVING Count(cs.OID) = Sum(RS.Check) 

使用下面的對象設置，是否有一個可以構建的等價的LINQ查詢，或者這是不可能的？注意道路類聲明中檢查的默認值。在下面的設置示例中，結果應該是零匹配。添加一個具有適當值的道路將使它返回一個。至少這是理想的。

我所遇到的問題是，它似乎這種類型的TSQL代碼只是太複雜，LINQ。我還沒有找到明顯或非顯而易見的解決方案來實現類似的行爲。因此，我認爲解決方案可能是停止嘗試複製SQL並做一些不同的事情。沒有足夠的LINQ經驗，我不知道從哪裏開始。

public class Roads : List<Road>{} 
public class Road 
{ 
    public int RID; 
    public int OID; 
    public int check = 1; 
} 
public class Cars : List<Car> { } 
public class Car 
{ 
    public int RID; 
    public int OID; 
} 

private void CheckCheck() 
{ 
    Roads rs = new Roads(); 
    Cars cs = new Cars(); 

    Car c = new Car(); 
    c.OID = 1; 
    c.RID = 1; 
    cs.Add(c); 
    c = new Car(); 
    c.OID = 1; 
    c.RID = 2; 
    cs.Add(c); 
    c = new Car(); 
    c.OID = 1; 
    c.RID = 3; 
    cs.Add(c); 

    Road r = new Road(); 
    r.OID = 1; 
    r.RID = 1; 
    rs.Add(r); 
    r = new Road(); 
    r.OID = 1; 
    r.RID = 2; 
    rs.Add(r); 

    // Results should be : 
    // OID where Count of OID from C = Count of OID from R 
} 

Answer 1

• 你HAVING子句會過濾掉不匹配的道路任何汽車。這使左連接成爲內連接。
• 你有COUNT(cs.OID)它說，這是計算汽車，但事實並非如此。您可能意味着COUNT(DISTINCT cs.OID)

這裏有一個直譯：

from c in Cars 
join r in Roads on new {c.OID, c.RID} equals new {r.OID, r.RID} 
group new {Car = c, Road = r} by c.OID into g 
let carCount = g.Count() //did you mean g.Select(x => x.Car.OID).Distinct().Count() 
let roadCount = g.Sum(x => x.Road.Check) 
where carCount = roadCount 
select new {OID = g.Key, CarCount = carCount, RoadCount = roadCount} 

---

目標是左表，返回每OID 1列，其中在記錄計數左表等於右表中匹配行的計數。

在此基礎上的描述，我會寫：

var carLookup = Cars.ToLookup(c => c.OID); 
var roadLookup = Roads.ToLookup(r => r.OID); 

from x in carLookup 
let carCount = x.Count() 
let roadCount = roadLookup[x.Key].Count() 
where carCount = roadCount 
select new {OID = g.Key, CarCount = carCount, RoadCount = roadCount} 

Answer 2

我不知道我完全理解所期望的結果是什麼，但這裏是我的投籃吧。對於LINQ來說，check字段並不是真正需要的，因爲除了計數之外，它沒有任何其他用途。

我誤解的加入，什麼HAVING條款做了查詢的重要性。現在回到原始的TSQL版本，但是計算結果已更改爲RoadCount。我相信這是你要求的。

var results = from Car in cs 
       join road in rs 
        on new { Car.OID, Car.RID } equals new { road.OID, road.RID } 
        into Roads 
       group roads by Car.OID into cars 
       let CarCount = cars.Count() 
       let RoadCount = cars.Sum(roads => roads.Count()) 
       where CarCount == RoadCount 
       select new 
       { 
        OID = cars.Key, 
        CarCount, 
        RoadCount 
       };