group by vs group join 生成的查询

Question

我有以下 group by linq 语句

from c in Categories
join p in Products on c equals p.Category into ps
select new { Category = new {c.CategoryID, c.CategoryName}, Products = ps };

但是，这会生成以下左外连接查询并返回所有类别，即使没有关联产品。

SELECT [t0].[CategoryID], [t0].[CategoryName], [t1].[ProductID], [t1].[ProductName],     [t1].[SupplierID], [t1].[CategoryID] AS [CategoryID2], [t1].[QuantityPerUnit],   [t1].[UnitPrice], [t1].[UnitsInStock], [t1].[UnitsOnOrder], [t1].[ReorderLevel], [t1].[Discontinued], (
    SELECT COUNT(*)
    FROM [Products] AS [t2]
    WHERE [t0].[CategoryID] = [t2].[CategoryID]
    ) AS [value]
FROM [Categories] AS [t0]
LEFT OUTER JOIN [Products] AS [t1] ON [t0].[CategoryID] = [t1].[CategoryID]
ORDER BY [t0].[CategoryID], [t1].[ProductID]

我真正想要的是只返回那些有相关产品的类别。但是如果我像这样重写 linq 查询：

from c in Categories
join p in Products on c equals p.Category
group p by new {c.CategoryID, c.CategoryName} into ps
select new { Category = ps.Key, Products = ps };

这给了我想要的结果，但是为每个类别生成了一个查询：

SELECT [t0].[CategoryID], [t0].[CategoryName]
FROM [Categories] AS [t0]
INNER JOIN [Products] AS [t1] ON [t0].[CategoryID] = [t1].[CategoryID]
GROUP BY [t0].[CategoryID], [t0].[CategoryName]
GO

-- Region Parameters
DECLARE @x1 Int SET @x1 = 1
DECLARE @x2 NVarChar(9) SET @x2 = 'Beverages'
-- EndRegion
SELECT [t1].[ProductID], [t1].[ProductName], [t1].[SupplierID], [t1].[CategoryID],  [t1].[QuantityPerUnit], [t1].[UnitPrice], [t1].[UnitsInStock], [t1].[UnitsOnOrder], [t1].[ReorderLevel], [t1].[Discontinued]
FROM [Categories] AS [t0]
INNER JOIN [Products] AS [t1] ON [t0].[CategoryID] = [t1].[CategoryID]
WHERE (@x1 = [t0].[CategoryID]) AND (@x2 = [t0].[CategoryName])
GO

-- Region Parameters
DECLARE @x1 Int SET @x1 = 2
DECLARE @x2 NVarChar(10) SET @x2 = 'Condiments'
-- EndRegion
SELECT [t1].[ProductID], [t1].[ProductName], [t1].[SupplierID], [t1].[CategoryID], [t1].[QuantityPerUnit], [t1].[UnitPrice], [t1].[UnitsInStock], [t1].[UnitsOnOrder], [t1].[ReorderLevel], [t1].[Discontinued]
FROM [Categories] AS [t0]
INNER JOIN [Products] AS [t1] ON [t0].[CategoryID] = [t1].[CategoryID]
WHERE (@x1 = [t0].[CategoryID]) AND (@x2 = [t0].[CategoryName])
GO

...

有没有一种方法可以执行相当于内部联接和分组依据的方法，并且仍然只产生一个像组联接这样的查询？

Answer 1

var queryYouWant =
  from c in Categories 
  join p in Products on c equals p.Category
  select new {Category = c, Product = p};

var result =
  from x in queryYouWant.AsEnumerable()
  group x.Product by x.Category into g
  select new { Category = g.Key, Products = g }; 

---

有没有一种方法可以执行相当于内部联接和分组依据的方法，并且仍然只产生一个像组联接这样的查询？

没有。当您说 GroupBy 后跟组元素的非聚合访问时，这就是使用组键作为过滤器的重复查询。

Answer 2

加入的目的是什么？

您的原始查询与此相同：

from c in Categories
select new { Category = new { c.CategoryID, c.CategoryName }, c.Products }

我是否遗漏了一些明显的东西？？？

如果您只需要包含产品的类别，请执行以下操作：

from c in Categories
where c.Products.Any()
select new { Category = new { c.CategoryID, c.CategoryName }, c.Products }

或者，如果您想展平结果：

from p in Products
select new { p, p.Category.CategoryID, p.Category.CategoryName }

后者将转换为内部联接或外部联接 - 取决于该关系是否可为空。您可以强制等效于内部连接，如下所示：

from p in Products
where p.Category != null
select new { p, p.Category.CategoryID, p.Category.CategoryName }