查询加入

Question

我通过LINQ声明

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 }; 

然而，这会产生以下左外连接查询，并返回即使没有相关的产品的所有类别有以下组。

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] 

我真正想要的只是返回那些有关联产品的类别。但是，如果我重新写LINQ查询，像这样：

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 }; 

这让我对每个类别生成一个查询所需的结果，但：

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 

... 

是否有办法做等效的内部连接和组合，并且仍然只产生一个单一的查询，如组连接？

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 }; 

---

有没有办法做一个内部的等效加入和组通过，仍然只能产生像一个单一查询该组加入？

不会。当您说GroupBy后跟非集合访问组元素时，这是一个重复的查询，其中组键是一个过滤器。

Answer 2

该连接的目的是什么？

你原来的查询与此：

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

上午我莫名其妙地失去了一些东西明显???

如果只想类别的产品，那么这样做：

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

或者，如果你想变平的结果：

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

后者将转化为内部或外部联接 - 取决于这种关系是否可以空。您可以强制内相当于加入如下：

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