LINQ TO SQL中的selectMany

首先看SelectMany的定义：

Queryable中的SelectMany 方法:将序列的每个元素投影到一个 IEnumerable<(Of <(T>)>) 并将结果序列组合为一个 IQueryable<(Of <(T>)>) 类型的序列。(引用MSDN)

在用LINQ TO SQL 来写查询语句时,有一个selectMany的语句,它标示着一对多的关系,这篇文章我想说下在LINQ TO SQL中几种可以等同selectMany的用法。

      系统转换成selectMany的条件:
          1:语句中不包含join ,into;
          2:需要2个以上的from:下面以两个表为例:如第一个表from c in 表1
   1):如果from的对象均用表名,(from c in 表2),则会转换成cross join;
             2):如果第二个表名以第一个表的子表形式出现,即类似c.表2,这又分两种情况,
                 1>:from o in c.表2,此时会形成inner join
     2>:from p in c.表2.DefaultIfEmpty(),此时会形成LEFT OUT JOIN

文中例子表结构说明:Customer表和Purchase表，通过ID与CustomerID建立关联。

CREATE TABLE [dbo].[Customer](
    [ID] [ int ] NOT NULL,
    [Name] [nvarchar]( 30 ) )

CREATE TABLE [dbo].[Purchase](
    [ID] [ int ] NOT NULL,
    [CustomerID] [ int ] NULL,
    [Date] [datetime] NOT NULL,
    [Description] [varchar]( 30 ) )

我们来实现SQL中的三种非常经典的联接方式。

第一:cross join,它的结果集是所有表的迪卡尔积。

// cross join
    from c in Customers
    from o in Purchases
    select o

在LINQ TO SQL中,下面的from都指定为表名的话,就会生成下面的语句:

SELECT [t1].[ID], [t1].[CustomerID], [t1].[Date], [t1].[Description], [t1].[Price]
FROM [Customer] AS [t0], [Purchase] AS [t1]

第二:inner join。

// inner join
    from c in Customers
    from o in c.Purchases
    select o

生成的SQL如下:

SELECT [t1].[ID], [t1].[CustomerID], [t1].[Date], [t1].[Description], [t1].[Price]
FROM [Customer] AS [t0], [Purchase] AS [t1]
WHERE [t1].[CustomerID] = [t0].[ID]

虽然没有显示的用inner join,但和它的功能是一样的.它的写法和上面的cross join看起来特别像,唯一的区别就在于cross join时,直接用了表名Purchases,而inner join用的时候变成了c.Pruchasex,即形成了一对多的情况。

第三: LEFT OUTER JOIN

from c in Customers
from p in c.Purchases.DefaultIfEmpty()
select new { c.Name, p.Description, Price = ( decimal ? ) p.Price }

生成的SQL如下:

SELECT [t0].[Name], [t1].[Description] AS [Description], [t1].[Price] AS [Price]
FROM [Customer] AS [t0]
LEFT OUTER JOIN [Purchase] AS [t1] ON [t1].[CustomerID] = [t0].[ID]

left outer join实际上是在inner join的基础上加了一个条件,利用DefaultIfEmpty(),当记录不匹配时,返回null
我们对上在的查询增加一个过滤条件。

from c in Customers
from p in c.Purchases.Where (p => p.Price > 1000 ).DefaultIfEmpty()
select new
{
    c.Name,
    p.Description,
    Price = ( decimal ? ) p.Price
}

对应的SQL：

SELECT [t0].[Name], [t1].[Description] AS [Description], [t1].[Price] AS [Price]
FROM [Customer] AS [t0]
LEFT OUTER JOIN [Purchase] AS [t1] ON ([t1].[Price] > @p0) AND ([t1].[CustomerID] = [t0].[ID])

此时上面的语句还是标准的LEFT OUT JOIN,如果我们改变下条件的位置呢?

from c in Customers
from p in c.Purchases.DefaultIfEmpty()
where p.Price > 1000
select new
{
    c.Name,
    p.Description,
    Price = ( decimal ? ) p.Price
}

对应的SQL：

SELECT [t0].[Name], [t1].[Description] AS [Description], [t1].[Price] AS [Price]
FROM [Customer] AS [t0]
LEFT OUTER JOIN [Purchase] AS [t1] ON [t1].[CustomerID] = [t0].[ID]
WHERE [t1].[Price] > @p0

条件改变位置后并没有改变join的本质,还是LEFT OUT JOIN,只不过查询的结果不一样了,从结果集上看,后面一种的效果和inner join的结果一样。

总结：上面的查询语句也可以用显示的join来查询，个人更喜欢用显示的join,因为相比较SQL更接近些，看起来要亲近些。在下篇文章中，我会总结显示用join查询的用法，其实最终的显示结果都一样，只是写法不同。

注:
本文示例均来自NutShell

转载于:https://www.cnblogs.com/Yjianyong/archive/2011/11/29/2267801.html

LINQ TO SQL中的selectMany(转)

LINQ TO SQL中的selectMany

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