【IT168技术文档】


  适用场景：在我们表关系中有一对一关系，一对多关系，多对多关系等。对各个表之间的关系，就用这些实现对多个表的操作。

  说明：在Join操作中，分别为Join(Join查询), SelectMany(Select一对多选择)和GroupJoin(分组Join查询)。
  该扩展方法对两个序列中键匹配的元素进行inner join操作
  SelectMany

  说明：我们在写查询语句时，如果被翻译成SelectMany需要满足2个条件。1：查询语句中没有join和into，2：必须出现EntitySet。在我们表关系中有一对一关系，一对多关系，多对多关系等，下面分别介绍一下。

to Many关系：
var q =
from c in db.Customers
from o in c.Orders
where c.City == "London"
select o;
语句描述：Customers与Orders是一对多关系。即Orders在Customers类中以EntitySet形式出现。所以第二个from是从c.Orders而不是db.Orders里进行筛选。
var q =
from p in db.Products
where p.Supplier.Country == "USA" && p.UnitsInStock == 0
select p;

  语句描述：这一句使用了p.Supplier.Country条件，间接关联了Supplier表。生成SQL语句为：

SELECT [t0].[ProductID], [t0].[ProductName], [t0].[SupplierID],
[t0].[CategoryID],[t0].[QuantityPerUnit],[t0].[UnitPrice],
[t0].[UnitsInStock], [t0].[UnitsOnOrder],[t0].[ReorderLevel],
[t0].[Discontinued] FROM [dbo].[Products] AS [t0]
LEFT OUTER JOIN [dbo].[Suppliers] AS [t1] ON
[t1].[SupplierID] = [t0].[SupplierID]
WHERE ([t1].[Country] = @p0) AND ([t0].[UnitsInStock] = @p1)
-- @p0: Input NVarChar (Size = 3; Prec = 0; Scale = 0) [USA]
-- @p1: Input Int (Size = 0; Prec = 0; Scale = 0) [0]

  Many to Many关系：

var q =
from e in db.Employees
from et in e.EmployeeTerritories
where e.City == "Seattle"
select new
{
e.FirstName,
e.LastName,
et.Territory.TerritoryDescription
};

  说明：多对多关系一般会涉及三个表(如果有一个表是自关联的，那有可能只有2个表)。这一句语句涉及Employees, EmployeeTerritories, Territories三个表。它们的关系是1：M：1。Employees和Territories没有很明确的关系。

  语句描述：这条生成SQL语句为：

SELECT [t0].[FirstName], [t0].[LastName], [t2].[TerritoryDescription]
FROM [dbo].[Employees] AS [t0] CROSS JOIN [dbo].[EmployeeTerritories]
AS [t1] INNER JOIN [dbo].[Territories] AS [t2] ON
[t2].[TerritoryID] = [t1].[TerritoryID]
WHERE ([t0].[City] = @p0) AND ([t1].[EmployeeID] = [t0].[EmployeeID])
-- @p0: Input NVarChar (Size = 7; Prec = 0; Scale = 0) [Seattle]

  3.自关联关系：

var q =
from e1 in db.Employees
from e2 in e1.Employees
where e1.City == e2.City
select new {
FirstName1 = e1.FirstName, LastName1 = e1.LastName,
FirstName2 = e2.FirstName, LastName2 = e2.LastName,
e1.City
};