IQueryable query = from c in ctx.Customers select c;
这样的查询句法不会导致语句立即执行，它仅仅是一个描述，对应一个SQL。仅仅在需要使用的时候才会执行语句，比如：
IQueryable query = from c in ctx.Customers select c;
foreach (Customer c in query)
Response.Write(c.CustomerID);
如果你执行两次foreach操作，将会捕获到两次SQL语句的执行：
IQueryable query = from c in ctx.Customers select c;
foreach (Customer c in query)
Response.Write(c.CustomerID);
foreach (Customer c in query)
Response.Write(c.ContactName);
对应SQL：
SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
SELECT [t0].[CustomerID], [t0].[CompanyName], [t0].[ContactName], [t0].[ContactTitle], [t0].[Address], [t0].[City], [t0].[Region], [t0].[PostalCode], [t0].[Country], [t0].[Phone], [t0].[Fax]
FROM [dbo].[Customers] AS [t0]
对于这样的需求，建议你先使用ToList()等方法把查询结果先进行保存，然后再对集合进行查询：
IEnumerable<Customer> customers = (from c in ctx.Customers select c).ToList();
foreach (Customer c in customers)
Response.Write(c.CustomerID);
foreach (Customer c in customers)
Response.Write(c.ContactName);
延迟执行的优点在于我们可以像拼接SQL那样拼接查询句法，然后再执行：
var query = from c in ctx.Customers select c;
var newquery = (from c in query select c).OrderBy(c => c.CustomerID);