LINQ左连接与分组并具有转换这个TSQL

Question

以下TSQL提供的我会如何解决这个问题，SQL的例子。目标是从左表中返回每个OID 1行，其中左表中的记录计数等于右表中匹配行的计数。

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) 

使用下面的对象设置，是否有一个可以构建的等价的LINQ查询，或者这是不可能的？注意道路类声明中检查的默认值。在下面的设置示例中，结果应该是零匹配。添加一个具有适当值的道路将使它返回一个。至少这是理想的。

我所遇到的问题是，它似乎这种类型的TSQL代码只是太复杂，LINQ。我还没有找到明显或非显而易见的解决方案来实现类似的行为。因此，我认为解决方案可能是停止尝试复制SQL并做一些不同的事情。没有足够的LINQ经验，我不知道从哪里开始。

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子句会过滤掉不匹配的道路任何汽车。这使左连接成为内连接。
• 你有COUNT(cs.OID)它说，这是计算汽车，但事实并非如此。您可能意味着COUNT(DISTINCT cs.OID)

这里有一个直译：

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} 

---

目标是左表，返回每OID 1列，其中在记录计数左表等于右表中匹配行的计数。

在此基础上的描述，我会写：

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结果是什么，但这里是我的投篮吧。对于LINQ来说，check字段并不是真正需要的，因为除了计数之外，它没有任何其他用途。

我误解的加入，什么HAVING条款做了查询的重要性。现在回到原始的TSQL版本，但是计算结果已更改为RoadCount。我相信这是你要求的。

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 
       };